The longtime Libyan leader vowed to defend his country from what he called "crusader aggression" and warned the involvement of international forces will subject the Mediterranean and North African region to danger and put civilians at risk.
The U.S. military said 112 Tomahawk cruise missiles were fired from American and British ships and submarines at more than 20 coastal targets to clear the way for air patrols to ground Libya's air force. French fighter jets fired the first salvos, carrying out several strikes in the rebel-held east.
President Barack Obama said military action was not his first choice.
"This is not an outcome the U.S. or any of our partners sought," Obama said from Brazil, where he is starting a five-day visit to Latin America. "We cannot stand idly by when a tyrant tells his people there will be no mercy."
Thousands of regime supporters, meanwhile, packed into the sprawling Bab al-Aziziya military camp in Tripoli where Gadhafi lives to protect against attacks.
The strikes, which were aimed at enforcing a U.N.-mandated no-fly zone, were a sharp escalation in the international effort to stop Gadhafi after weeks of pleading by the rebels who have seen early gains reversed as the regime unleashed the full force of its superior air power and weaponry.
Gadhafi, who has ruled Libya for 41 years, said in a telephone call to Libyan state TV that he was opening weapons depots to allow his people to arm themselves in defense.
He said the international action against his forces was unjustified, calling it "simply a colonial crusader aggression that may ignite another large-scale crusader war."
His regime also acted quickly in the run-up to the strikes, sending warplanes, tanks and troops into the eastern city of Benghazi, the rebel capital and first city to fall to the rebellion that began Feb. 15. Then the government attacks appeared to go silent.
Operation Odyssey Dawn, as the coalition operation has been dubbed, followed an emergency summit in Paris during which the 22 leaders and top officials agreed to do everything necessary to make Gadhafi respect a U.N. Security Council resolution Thursday calling for the no-fly zone and demanding a cease-fire, French President Nicolas Sarkozy said.
"Our consensus was strong, and our resolve is clear. The people of Libya must be protected, and in the absence of an immediate end to the violence against civilians our coalition is prepared to act, and to act with urgency," Obama said earlier.
Navy Vice Adm. William E. Gortney, director of the Pentagon's Joint Staff, told reporters in Washington that U.S. ships and a British submarine had launched the first phase of a missile assault on Libyan air defenses to clear the way for the imposition of a U.N.-mandated no-fly zone over the North African country.
Gortney said the mission has two goals: prevent further attacks by Libyan forces on rebels and other civilians, and degrade the Libyan military's ability to contest a no-fly zone.
Mohammed Ali, a spokesman for the exiled opposition group the Libyan Salvation Front, said the Libyan air force headquarters at the Mateiga air base in eastern Tripoli, and the Aviation Academy in Misrata had been targeted.
About 20 French fighter jets carried out "several strikes" earlier Saturday, military spokesman Thierry Burkhard told The Associated Press. He said earlier that one of the planes had fired the first shot against a Libyan military vehicle.
"All our planes have returned to base tonight," he said, and denied a Libyan TV report that a French plane had been hit.
He would not elaborate on what was hit or where, but said French forces are focusing on the Benghazi area and U.S. forces are focused in the west.
The U.S. has struck Libya before. Former President Reagan launched U.S. airstrikes on Libya in 1986 after a bombing at a Berlin disco -- which the U.S. blamed on Libya -- that killed three people, including two American soldiers. The airstrikes killed about 100 people in Libya, including Gadhafi's young adopted daughter at his Tripoli compound.
Libyan regime official Mohammed al-Zwei claimed a large number of civilians were injured when several civilian and military sites in the capital, Tripoli, and the nearby city of Misrata were hit.
"This barbaric aggression against the Libyan people comes after we had announced a cease-fire against the armed militias which are part of al-Qaida in the Islamic Maghreb," he said. The targets could not be independently verified.
The rebels said earlier that they had hoped for more, sooner from the international community, after a day when crashing shells shook the buildings of Benghazi and Gadhafi's tanks rumbled through the university campus.
"People are disappointed, they haven't seen any action yet. The leadership understands some of the difficulties with procedures but when it comes to procedures versus human lives the choice is clear," said Essam Gheriani, a spokesman for the opposition. "People on the streets are saying where are the international forces? Is the international community waiting for the same crimes to be perpetrated on Benghazi has have been done by Gadhafi in the other cities?"
A doctor said 27 bodies had reached hospitals by midday. As night fell, though, the streets grew quiet.
Libyan state television also showed Gadhafi supporters converging on the international airport and a military garrison in Tripoli, and the airport in Gadhafi's hometown of Sirte, in an apparent attempt to deter bombing.
In an open letter, Gadhafi warned: "You will regret it if you dare to intervene in our country."
In Paris, U.S. Secretary of State Hillary Rodham Clinton said Gadhafi's government had lost all legitimacy and lied about the cease-fire.
"We have every reason to fear that left unchecked, Gadhafi will commit unspeakable atrocities," she said.
Saturday's emergency meeting in Paris, which also included U.N. Secretary-General Ban Ki-Moon, Arab League Secretary-General Amr Moussa and the foreign ministers of Jordan, Morocco and the United Arab Emirates, was the largest international military action since the beginning of the Iraq war, launched almost exactly eight years ago.
"The time for action has come, it needs to be urgent," British Prime Minister David Cameron said after the summit.
Earlier Saturday, a plane was shot down over the outskirts of Benghazi, sending up a massive black cloud of smoke. An Associated Press reporter saw the plane go down in flames and heard the sound of artillery and crackling gunfire.
Before the plane went down, journalists heard what appeared to be airstrikes from it. Rebels cheered and celebrated at the crash, though the government denied a plane had gone down -- or that any towns were shelled on Saturday.
The fighting galvanized the people of Benghazi, with young men collecting bottles to make gasoline bombs. Some residents dragged bed frames and metal scraps into the streets to make roadblocks.
"This city is a symbol of the revolution, it's where it started and where it will end if this city falls," said Gheriani.
But at Jalaa hospital, where the tile floors and walls were stained with blood, the toll was clear.
"There are more dead than injured," said Dr. Ahmed Radwan, an Egyptian who had been there helping for three weeks.
Jalaa's Dr. Gebreil Hewadi, a member of the rebel health committee, said city hospitals had received 27 bodies.
At a news conference in the capital, Tripoli, the government spokesman read letters from Gadhafi to Obama and others involved in the international effort.
"Libya is not yours. Libya is for the Libyans. The Security Council resolution is invalid," he said in the letter to Sarkozy, British Prime Minister David Cameron, and U.N. Secretary-General Ban Ki-Moon.
To Obama, the Libyan leader was slightly more conciliatory: "If you had found them taking over American cities with armed force, tell me what you would do."
In a joint statement to Gadhafi late Friday, the United States, Britain and France -- backed by unspecified Arab countries -- called on Gadhafi to end his troops' advance toward Benghazi and pull them out of the cities of Misrata, Ajdabiya and Zawiya. It also called for the restoration of water, electricity and gas services in all areas. It said Libyans must be able to receive humanitarian aid or the "international community will make him suffer the consequences" with military action.
Foreign Minister Moussa Koussa said that Libyan officials had informed the U.N. and the Security Council that the government was abiding by the cease-fire it had announced Friday and called for a team of foreign observers to verify that.
In the course of the rebellion, Libya has gone from a once-promising economy with the largest proven oil reserves in Africa to a country in turmoil. The foreign workers that underpinned the oil industry have fled; production and exports have all but ground to a halt; and its currency is down 30 percent in just two weeks.
The oil minister, Shukri Ghanem, held a news conference calling on foreign oil companies to send back their workers. He said the government would honor all its contracts.
"We are still considering all our contracts and agreements with the oil companies valid," he said. "We hope from their part that they will honor their agreements, that they will send back their experts and their people to work."
He suggested future decisions on oil deals would favor countries that did not join the international force against Gadhafi: "A friend in need is a friend indeed," he told reporters in Tripoli.
Italy, which had been the main buyer for Libyan oil, offered the use of seven air and navy bases already housing U.S., NATO and Italian forces to enforce the no-fly zone over Libya.
Italy's defense minister, Ignazio La Russa, said Saturday that Italy wasn't just "renting out" its bases for others to use but was prepared to offer "moderate but determined" military support.
Warplanes from the United States, Canada, Denmark arrived at Italian air bases Saturday as part of an international military buildup. Germany backed the operation but isn't offering its own forces.