Authorities have not released information on injuries from Tuesday's clash. However, officials at Highland Hospital in Oakland said today that one protester, 24-year-old Scott Olsen of Daly City, a member of Iraq Veterans Against the War, was in critical condition.

The antiwar group said Olsen, a system administrator at a San Francisco software firm, suffered a skull fracture when he was hit by a "blunt object." Olsen joined the U.S. Marines in 2006, served two tours in Iraq, and was discharged in 2010, the group said.

Video footage widely distributed on the Internet shows a protester, identified by the antiwar group as Olsen, being carried away by others with a head wound. While he lay wounded, the footage appears to show an officer tossing something - perhaps a tear gas canister - toward people trying to help him.

"I think it is a sad state of affairs when a Marine can't assemble peacefully in the streets without getting injured," said Jose Sanchez, executive director of Iraq Veterans Against the War. "We are pretty upset about it."