Scott Olsen, 24, suffered a fractured skull Tuesday in a march with other protesters toward City Hall, said Dottie Guy, of the Iraq Veterans Against the War. The demonstrators had been making an attempt to re-establish a presence in the area of a disbanded protesters' camp when they were met by officers in riot gear.
Several small skirmishes broke out and officers cleared the area by firing tear gas after police say protesters threw rocks and bottles at them.
It's not known exactly what type of object struck Olsen in the chaos, though Guy's group alleges it was a police projectile. Multiple attempts to reach Oakland police by The Associated Press were unsuccessful ahead of a late afternoon news conference.
Guy said also it wasn't immediately clear whether Olsen, a network administrator in Daly City, would need surgery.
"It's still too early to tell," Guy said. "We're hoping for the best."
Curt Olsen, a spokesman for Highland Hospital in Oakland, confirmed that the veteran was in critical condition but could not release any more information.
The clash Tuesday evening came as officials complained about what they described as deteriorating safety, sanitation and health issues at the dismantled camp.
The same concerns were being raised by San Francisco officials who warned protesters Wednesday that they could face arrest if they continue camping in a city plaza. In a letter, Police Chief Greg Suhr said the protesters could be arrested for violating a variety of city laws against camping, cooking, urinating and littering in public parks.
"Existing and ongoing violations make you subject to arrest," Suhr wrote in the notice, but didn't say if or when arrests would occur.
Police have taken down a previous Occupy San Francisco camp in the Justin Herman Plaza and also cleared another camp outside the Federal Reserve Bank downtown.
Olsen, who completed his service last year, participated in the protest in Oakland because he felt corporations and banks have too much influence on the government, Guy said.
A vigil for him is scheduled to be held Wednesday evening near the Oakland City Hall, she said.
Oakland demonstrators vowed on Wednesday to return to their protest site just hours after police cleared hundreds of people from the streets with tear gas and bean bag rounds.
A Twitter feed used by Oakland's Occupy Wall Street movement called on protesters to return to downtown at 6 p.m. for another round, and some demonstrators vowed to return as soon as possible.
Max Alper, 31, a union organizer from Berkeley, gathered with a handful of other protesters Wednesday at the scene of Tuesday night's clash.
"As soon as these barricades are moved, hundreds of people are going to come back. These actions by police were wrong, but they're just going to strengthen the movement," Alper said.
Alper was arrested Tuesday morning when he went to witness the police raid on the Occupy Oakland encampment outside City Hall, he said. He said his arm was injured when baton-swinging police descended on him and other protesters.
Police estimated that there were roughly 1,000 demonstrators at the first clash following the march.
City officials say that two officers were injured. At least five protesters were arrested and several others injured in the evening clashes.
Associated Press reporters Jason Dearen and Marcus Wohlsen contributed to this report.