An Iraq war veteran has a fractured skull and brain swelling after allegedly being hit by a police projectile.
Scott Olsen is in a "critical condition" in Highland hospital in Oakland, a hospital spokesman confirmed.
Olsen, 24, suffered the head injury during protests in Oakland on Tuesday evening. More than 15 people were arrested after a crowd gathered to demonstrate against the police operation to clear two Occupy Oakland camps in the early hours of Tuesday morning.
Jay Finneburgh, a photographer who was covering the protest, published pictures of Olsen lying on the ground.
"This poor guy was right behind me when he was hit in the head with a police projectile. He went down hard and did not get up," Finneburgh wrote.
Olsen was taken to Highland by fellow protesters.
The Guardian spoke to people with Olsen at the hospital. Adele Carpenter, who knows Olsen through his involvement with anti-war groups, said she arrived at the hospital at 11pm on Tuesday night.
Carpenter said she was told by a doctor at the hospital that Olsen had a skull fracture and was in a "serious but stable" condition. She said he had been sedated and was unconscious.
"I'm just absolutely devastated that someone who did two tours of Iraq and came home safely is now lying in a US hospital because of the domestic police force," Carpenter said.
Olsen had only moved to Oakland in July, Carpenter said. He is a member of Veterans for Peace and Iraq Veterans Against the War, and met Carpenter through her work with the civilian soldier alliance.
Keith Shannon, who served with Olsen in Iraq, arrived at the hospital after protesters contacted him through Facebook. He confirmed Olsen had a fractured skull, and said he had been told by a doctor Olsen also had brain swelling.
A neurosurgeon was due to assess Olsen to determine if he needed surgery, Shannon said.
"It's really hard," Shannon said. "I really wish I had gone out with him instead of staying home last night."
Shannon, who is also 24, said he had seen the video footage showing Olsen lying on the floor as a police officer throws an explosive device near him.
"It's terrible to go over to Iraq twice and come back injured, and then get injured by the police that are supposed to be protecting us," he said.
He said Olsen had served two tours of Iraq, in 2006 and 2007. Olsen was in 3rd Battalion, 4th Marines with Shannon before leaving the military in 2010.
He moved to the San Francisco area in July and works for Opswat, a software company, living with Shannon in Daly City, just south of San Francisco.
Shannon said Olsen was hit in the head by a tear gas canister or smoke canister shot by a police officer. He said Olsen had a curved scar on his forehead consistent with a canister.
Protesters who had accompanied Olsen to Highland hospital got in touch with Shannon through Facebook, after Olsen said he lived with someone called "Keith". Shannon said he was told Olsen was unable to say his surname.
Olsen is originally from Wisconsin and some of his family were planning to fly out to California to be with him, Shannon said.
Video footage published to YouTube shows Olsen lying prone in front of a line of police. Around 10 people gather around him in an apparent attempt to provide aid, before a police officer throws an explosive device into their midst, scattering the group.
Footage captured after the explosion, which appears to be from a flash bang grenade, shows Olsen being carried away by a group of people.
Oakland police confirmed at a press conference that they used tear gas and baton rounds, but said they did not use flash bang grenades. Police could not be immediately reached for comment.
