Occupy Oakland: Scott Olsen, an Iraq veteran hospitalized in serious condition.

Scott Olsen, an Iraq war veteran was critically injured by a projectile during Tuesday night clash between Occupy Oakland protesters and police.

Scott Olsen, 24, from Daly City, California, was hospitalized in serious condition after he suffered a fractured skull in a march with other protesters toward City Hall.

In New York, an impromptu Occupy Wall Street march caused widespread disruption last night as it spread across the city, causing police to shut several streets and arrest some protesters.

The New York march was to show solidarity with Occupy Oakland protesters after police in the Californian city were accused of using unnecessary force by unleashing tear gas on a crowd on Tuesday evening.

Several hundred protesters began marching from Zuccotti Park, the base of the Occupy Wall Street movement, towards City Hall Park which was cordoned off.

Occupy Wall Street demonstrators broke off into several smaller groups, some of whom confronted police. At some intersections, marchers were met with a line of police motorcycles or mesh barriers blocking access to the road.

The march ended peacefully shortly after 11:00 p.m., as the largest group of protesters walked back to Zuccotti Park, followed by a line of NYPD patrol cars.

On Tuesday night, Occupy Oakland 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 as tensions grew over demonstration encampments across the San Francisco Bay Area.

For the moment it’s not known exactly what type of object struck veteran Scott Olsen or who might have thrown it, though Guy’s group said it was lodged by officers. Several small skirmishes had broken out in the night with police clearing the area by firing tear gas and protesters throwing rocks and bottles at them.

During a news conference, Police Chief Howard Jordan said that the events leading up to Scott Olsen’s injury would be investigated as vigorously as a fatal police shooting.

“It’s unfortunate it happened. I wish that it didn’t happen. Our goal, obviously, isn’t to cause injury to anyone.”

According to an Oakland hospital spokesman, Scott Olsen, a network administrator in Daly City, was in critical condition on Wednesday.

Scott Olsen, who was born in Onalaska, Wisconsin, is an Iraq Veterans Against the War member and served in 3rd Battalion, 4th Marines.

He served in Iraq in 2006 and 2007 before leaving the military last year.

“Oakland Police Department fired a tear gas canister at his head, fracturing his skull,” said Scott Olsen’s friend Joshua Shepherd.

Scott Olsen was being treated for injuries sustained during the protest at Highland General Hospital.

Oakland city officials later allowed protesters back into the plaza where their 15-day-old encampment had been raided, but said they would continue prohibiting people from spending the night there.

Authorities had announced on Tuesday night over a loudspeaker that those who refused to leave could be targeted by “chemical agents”. One woman in a wheelchair, who was struggling to breathe, had to be helped away by friends while another woman, hit by a canister, was carried off.