"I heard some commotion going on. Took a look out my window saw an arrest being made. Got out of my car and starting rolling film with my iPhone," said Morgan Manos, a San Francisco Uber driver.
Manos says that Chamberlain called out to him for help as officers took him away.
"The cop said back away. The guy who was getting arrested looked at me and said help," said Manos.
Chamberlain, 42, once worked with San Francisco's political elite. He was a social media consultant who had worked with Gavin Newsom and was well-known in the city's political circles.
He became the target of a manhunt Saturday after FBI agents raided his home near the intersection of Polk and Jackson streets. Officials say they found explosive devices in his apartment but Chamberlain had disappeared.
About 2 p.m. Monday, a tip led FBI agents to the Mad Dog in the Fog bar in San Francisco's Lower Haight neighborhood. The bartender said Chamberlain used his ATM card.
FBI agents followed the lead but Chamberlain was not there when they arrived.
Mike Burt, an employee at the nearby Toronado Bar across the street told KTVU, "There was a couple undercover officers with a picture, 3 pictures of the guy asking if anybody seen him."
At 6:22 p.m. the officers had located Chamberlain and arrested him. San Francisco police brought in the bomb squad. They used a robot to check the vehicle for explosives.
"I figured if there's a manhunt surely this guy's going to get out of the city. I didn't imagine I'd run into him down at the beach," said Manos.
Chamberlain is being held on federal charges of possessing explosives.
The FBI and police plan to have news conference Tuesday morning with more details.