The violent wall of water swept away houses, cars and ships. Fires burned out of control. Power to cooling systems at two nuclear power plants was knocked out, forcing thousands of nearby residents to be evacuated. A boat was caught in the vortex of a whirlpool at sea.
The death toll rose steadily throughout the day, but the true extent of the disaster was not known because roads to the worst-hit areas were washed away or blocked by debris and airports were closed.
After dawn Saturday, the scale of destruction became clearer. Aerial scenes of the town of Ofunato showed homes and warehouses in ruins. Sludge and high water spread over acres of land, with people seeking refuge on roofs of partially submerged buildings. At one school, a large white "SOS" had been spelled out in English.
Chief Cabinet Secretary Yukio Edano said an initial assessment found "enormous damage," adding that the Defense Ministry was sending troops to the hardest-hit region.
President Barack Obama pledged U.S. assistance following what he called a potentially "catastrophic" disaster. He said one U.S. aircraft carrier is already in Japan and a second was on its way. A U.S. ship was also heading to the Marianas Islands to assist as needed, he added.
The entire Pacific had been put on alert -- including coastal areas of South America, Canada and Alaska -- but waves were not as bad as expected.
The magnitude-8.9 offshore quake struck at 2:46 p.m. local time and was the biggest to hit Japan since record-keeping began in the late 1800s. It ranked as the fifth-largest earthquake in the world since 1900 and was nearly 8,000 times stronger than one that devastated Christchurch, New Zealand, last month, scientists said.
The quake shook dozens of cities and villages along a 1,300-mile (2,100-kilometer) stretch of coast and tall buildings swayed in Tokyo, hundreds of miles from the epicenter. Prime Minister Naoto Kan was attending a parliamentary session at the time.
"I thought I was going to die," said Tokyo marketing employee Koto Fujikawa. "It felt like the whole structure was collapsing."
Fujikawa, 28, was riding a monorail when the quake hit and had to later pick her way along narrow, elevated tracks to the nearest station.
Minutes later, the earthquake unleashed a 23-foot (seven-meter) tsunami along the northeastern coast of Japan near the coastal city of Sendai in Miyagi prefecture. The U.S. Geological Survey said that after the intial huge quake, there were 123 aftershocks off Japan's main island of Honshu, 110 of them of magnitude 5.0 or higher
Large fishing boats and other vessels rode the high waves ashore, slamming against overpasses or scraping under them and snapping power lines along the way. A fleet of partially submerged cars bobbed in the water. Ships anchored in ports crashed against each other.
The tsunami roared over embankments, washing anything in its path inland before reversing direction and carrying the cars, homes and other debris out to sea. Flames shot from some of the homes, apparently from burst gas pipes.
Waves of muddy waters flowed over farms near Sendai, carrying buildings, some of them ablaze. Drivers attempted to flee. The tarmac at Sendai's airport was inundated with thick, muddy debris that included cars, trucks, buses and even light planes.
Highways to the worst-hit coastal areas buckled. Telephone lines snapped. Train service was suspended in northeastern Japan and in Tokyo, which normally serves 10 million people a day. Untold numbers of people were stranded in stations or roaming the streets. Tokyo's Narita airport was closed indefinitely.
Police said 200-300 bodies were found in Sendai, although the official casualty toll was 185 killed, 741 missing and 948 injured.
A ship with 80 dock workers was swept away from a shipyard in Miyagi prefecture. All on the ship was believed to be safe, although the vessel had sprung a leak and was taking on some water, Japan's coast guard said.
In the coastal town of Minami-soma, about 1,800 houses were destroyed or ravaged, a Defense Ministry spokeswoman said. Fire burned well past dark in a large section of Kesennuma, a city of 70,000 people in Miyagi.
A resident in Miyagi who had been stranded on his roof, surrounded by water, mud and fallen trees, was rescued by a Self-Defense Force helicopter Saturday morning, TV video showed.
Japan declared its first-ever states of emergency for five nuclear reactors at two power plants after the units lost cooling ability in the aftermath of the earthquake, and workers struggled to prevent meltdowns.
The earthquake knocked out power at the Fukushima Daiichi plant, and because a backup generator failed, the cooling system was unable to supply water to cool the 460-megawatt No. 1 reactor. Although a backup cooling system is being used, Japan's nuclear safety agency said pressure inside the reactor had risen to 1.5 times the level considered normal.
Authorities said radiation levels had jumped 1,000 times normal inside Unit 1 and were measured at eight times normal outside the plant. They expanded an earlier evacuation zone more than threefold, from 3 kilometers to 10 kilometers (2 miles to 6.2 miles). About 3,000 people were urged to leave their homes in the first announcement.
Tokyo Electric Power Co. warned of power shortages and an "extremely challenging situation in power supply for a while."
The utility, which also operates reactors at the nearby Fukushima Daini plant, later confirmed that cooling ability had been lost at three of four reactors there, as well as a second Fukushima Daiichi unit. The government promptly declared a state of emergency there as well.
The level outside the 40-year-old plant in Onahama, a city about 170 miles (270 kilometers) northeast of Tokyo, is still considered very low compared to the annual exposure limit, said Ryohei Shiomi, an official with the Japan Nuclear and Industrial Safety Agency. It would take 70 days of standing at the gate to reach the limit, he said.
The Defense Ministry said it had sent troops trained to deal with chemical disasters to the plants in case of a radiation leak.
An American working at the Fukushima Daiichi facility said the whole building shook and debris fell from the ceiling. Danny Eudy, 52, a technician employed by Texas-based Atlantic Plant Maintenance, and his colleagues escaped the building just as the tsunami hit, his wife told The Associated Press.
"He walked through so much glass that his feet were cut. It slowed him down," said Pineville, Louisiana, resident Janie Eudy, who spoke to her husband by phone after the quake.
The group watched homes and vehicles carried away in the wave and found their hotel mostly destroyed when they reached it.
A large fire erupted at the Cosmo oil refinery in the city of Ichihara and burned out of control with 100-foot (30-meter) flames whipping into the sky.
Also in Miyagi prefecture, a fire broke out in a turbine building of a nuclear power plant, but it was later extinguished, said Tohoku Electric Power Co.
Japanese automakers Toyota, Nissan and Honda halted production at some assembly plants in areas hit by the quake. One worker was killed and more than 30 injured after being crushed by a collapsing wall at a Honda Motor Co. research facility in northeastern Tochigi prefecture, the company said.
Jesse Johnson, a native of Nevada who lives in Chiba, north of Tokyo, was eating at a sushi restaurant with his wife when the quake hit.
"At first it didn't feel unusual, but then it went on and on. So I got myself and my wife under the table," he told the AP. "I've lived in Japan for 10 years, and I've never felt anything like this before. The aftershocks keep coming. It's gotten to the point where I don't know whether it's me shaking or an earthquake."
Tokyo was brought to a near standstill. Tens of thousands of people were stranded with the rail network down, and the streets were jammed with cars, buses and trucks trying to get out of the city.
The city set up 33 shelters in city hall, on university campuses and in government offices, but many planned to spend the night at 24-hour cafes, hotels and offices.
NHK said more than 4 million buildings were without power in Tokyo and its suburbs.
Jefferies International Ltd., a global investment banking group, estimated overall losses of about $10 billion.
The tsunami hit Hawaii before dawn Friday, with most damage coming on the Big Island. The waves covered beachfront roads and rushed into hotels. One house was picked up and carried out to sea. Low-lying areas in Maui were flooded by 7-foot waves.
On the U.S. mainland, marinas and harbors in California and Oregon bore the brunt of the damage, estimated by authorities to be in the millions of dollars. Boats crashed into each other in marines and some vessels were washed out to sea.
Rescue crews were searching for a man who was swept away in northern California while taking pictures. Two people with him tried to rescue him, although they were able to return to shore.
Thousands fled homes in Indonesia after officials warned of a tsunami up to 6 feet (2 meters) high, but waves of only 4 inches (10 centimeters) were measured. No big waves came to the Northern Mariana Islands, a U.S. territory, either.
Islands across the South Pacific were hit by bigger-than-normal waves, but no major damage was reported. Surges up to 26 inches (66 centimeters) high were reported in American Samoa, Nauru, Saipan and at the far northern tip of New Zealand.
In Tonga, water flooded houses in the low-lying Ha'apai islands early Saturday, police said. Thousands in the capital, Nuku'alofa, sought refuge at the king's residence on higher ground, Radio Tonga said.
The quake struck at a depth of six miles (10 kilometers), about 80 miles (125 kilometers) off Japan's east coast, the USGS said. The area is 240 miles (380 kilometers) northeast of Tokyo. Several quakes hit the same region in recent days, including one measured at magnitude 7.3 on Wednesday that caused no damage.
"The energy radiated by this quake is nearly equal to one month's worth of energy consumption" in the United States, USGS scientist Brian Atwater told The Associated Press.
Early Saturday, a magnitude-6.6 earthquake struck the central, mountainous part of Japan -- far from the original quake's epicenter. It was not immediately clear if that temblor was related to the others.
Japan's worst previous quake was a magnitude 8.3 in Kanto that killed 143,000 people in 1923, according to USGS. A 7.2-magnitude quake in Kobe killed 6,400 people in 1995.
Japan lies on the "Ring of Fire" -- an arc of earthquake and volcanic zones stretching around the Pacific where about 90 percent of the world's quakes occur, including the one that triggered the Dec. 26, 2004, Indian Ocean tsunami that killed an estimated 230,000 people in 12 countries. A magnitude-8.8 temblor that shook central Chile in February 2010 also generated a tsunami and killed 524 people.