The gunman got into a truck and drove away from Salon Meritage after opening fire. He was stopped by officers about a half-mile away and surrendered without incident while saying he had multiple weapons with him, police Sgt. Steve Bowles said.
His name and those of the victims were not immediately released.
Bowles initially said, "There may be something to the motive as to a relationship with somebody in the salon, that is our assumption."
Later, he said he couldn't comment further about the motive.
Kimberly Criswell, who owns a salon just two doors down from the scene of the shootings, said she and her customers and employees heard gunshots, and her receptionist saw a man through a window as he was shot in a parking lot.
"There was like a pop, pop and my receptionist screamed, `he just shot that man' and we all ran into the bathroom and locked the door,"' Criswell said.
Glenn Zachman, who owns a video news-gathering service, said he arrived at the scene of the arrest shortly after police and saw they had placed plastic bags over the man's hands to preserve possible gunshot residue.
He also saw a bulletproof vest on the back of a patrol car but didn't know if the man, dressed in a button-down shirt and pants, had been wearing it.
The suspect was cooperative when officers, working from a description of the shooter, stopped him near the salon, Bowles said.
TV news video showed the man, in handcuffs, being placed in a patrol car and taken away about two-and-a-half hours after the shooting. A new white pickup truck that was believed to be his was parked on the modest residential street with its doors open.
Earlier, police responding to a report of shots fired found six people dead and three wounded. Two of the wounded died at a hospital. The other person was listed in critical condition.
Bowles said the victims were scattered throughout the salon. One wounded person, a man, was found outside the building. It wasn't known if he was the one person who survived.
Police, meanwhile, were still trying to determine the sequence of events inside the shop.
"We're unsure at this point if he shot from the entrance and people, as they were shot, ran in seeking cover or seeking shelter, but we have fatalities throughout the salon," Bowles told reporters at a news conference outside the business.
"From my observation, it did look like people were seeking shelter at the time," he said.
Bowles said the salon was busy at the time, with every hair-dressing station in operation.
He didn't know what type of weapon was used or if the man used more than one.
Salon employee Lorainne Bruielle, who wasn't working Wednesday, told the Long Beach Press-Telegram the gunman was the husband of another employee.
Bruielle said she talked to the husband of one of the employees involved, who said one worker locked herself in the salon's facial room and was unharmed while another man locked himself in a bathroom but was wounded.
Several visibly distressed people stood near the scene talking on cellphones or to police shortly after the shootings. They declined to be interviewed.
About 41/2 hours later, a police officer escorted a woman who was crying and shaking past crime-scene tape and to her car.
Cindy Spinosa, 51, who works at a nearby business, said she heard a siren around the time of the shooting.
"After that one of my co-workers got a phone call from her nephew," Spinosa said. "He was outdoors when the suspect got into his truck and took off.
"It's a little disarming," she said. "This is such a quiet community. We don't expect things like this."
Relatives of victims were taken to a nearby spiritual center.
Seal Beach has seen just one other homicide in the past four years. The killings on Wednesday stunned the normally quiet community of about 25,000 that boasts on its website that it has "retained its quaint, small-town atmosphere" since it was founded in 1915.
"Seal Beach is a small, safe community. We don't experience these things, ever," Bowles said.
The beachfront city is home to Leisure World, a gated senior citizen community of 9,000 people, as well as the Seal Beach Naval Weapons Station military complex. Two-thirds of the city's 13.23 square miles are occupied by the Seal Beach National Wildlife Refuge.
The city is also home to numerous salons, Criswell said, and most employees know each other.
"A lot of these people have worked together for years and they loved working there," she said of the people at Salon Meritage.
The Associated Press contributed to this report.