Zachary Treisman didn't have any idea where his Friday evening bike ride would end. That's not unusual, since Critical Mass, the bike demonstration Treisman rode in, starts every month without any planned route or ride leaders.

But Treisman's ride ended the last place he intended — the King County Jail, where he was booked on suspicion of assault.

Treisman, 30, and another rider, 18-year-old Jason Brien, both of Seattle, were arrested by King County sheriff's detectives after Brien blocked a downtown intersection to allow hundreds of bikers to stick together through changing traffic signals.

Critical Mass participants said the detectives, wearing plain clothes and driving an unmarked van, appeared to be motorists with a case of "road rage" over the traffic delay. A King County sheriff's spokesman said the four detectives identified themselves as law-enforcement officers before arresting the men.

Blocking intersections with bikes is a method routinely used to keep riders safe during the monthly demonstration for bike awareness and safety, said Treisman's attorney, David Speikers.

Speikers said his client, a University of Washington graduate student, was trying to defend Brien and didn't know the men were sheriff's detectives until he was on the ground being handcuffed.

"They started the fight as citizens, and they ended the fight as law enforcement," Speikers said. "When you don't have properly attired uniforms and a marked vehicle, and you start doing things like that, it's very easy to assume you are not a police officer."

Deputy Rodney Chinnick, a sheriff's spokesman, said the detectives gave Brien at least three opportunities to move out of the road, used lights and sirens, and clearly identified themselves.

Chinnick said Brien was "intentionally disrupting the flow of traffic" and Treisman assaulted a plain-clothes detective who was arresting Brien.

"If he'd gotten out of the middle of the road, that would've ended it," Chinnick said.

When the first detective got out of the van, Chinnick said, Brien turned and ran into the crowd but was caught by another detective. Treisman came up behind that detective and put him in a choke-hold, Chinnick said.

The Sheriff's Office contends Treisman continued to fight when the other detectives intervened, punching one of them even though they were yelling, "Police, stop resisting."

Treisman said he stopped fighting as soon as the detectives identified themselves.

Both Brien and Treisman were jailed Friday night. Brien was arrested on suspicion of disorderly conduct and being a minor in possession of alcohol. He was released without bail early Saturday, jail records show. Treisman was booked on suspicion of assault and posted $3,000 bail on Saturday.

Brien could not be reached for comment.

Written statements from several other bike riders concur with Treisman's account, saying the detectives wrestled the men to the ground before identifying themselves as police officers.

Riders also said Seattle police usually interact peacefully with the demonstrators, even when they block intersections.

"Usually they ignore us, or smile and wave," said Joby Lafky, who has been riding with Critical Mass for two years. "Occasionally a police car will ride with us at the back of the group."

Seattle police spokesman Sean Whitcomb, who referred questions about Friday's incident to the Sheriff's Office, said he wasn't aware of any problems, arrests or citations involving demonstrators involved in Critical Mass rides in recent years.

He declined to say how Seattle police typically respond to the monthly bike rides, because the department does not publicly comment on police tactics. But police consider the rides "planned events," Whitcomb said.

"We know that they hold their demonstrations monthly ... and we'll count on another one at the end of the month," he said.

Joe Mullin: 206-464-2761 or jmullin@seattletimes.com