Police have fired tear gas at protesters from the "Occupy Oakland" movement in the US city of Oakland in California.
Thursday's clashes followed a peaceful rally which shut down Oakland's port, the fifth busiest cargo facility in the country.
Protesters called for a general strike to protest economic conditions and police brutality.
An estimated 5,000 people gathered for the demonstration against what they call corporate greed.
The port, which handles about $39bn a year in imports and exports, stopped operations after being swarmed by thousands of the marchers.
While the city was not shut down by the protest, as had been the aim of the demonstrators, many businesses chose to remain closed on Wednesday.
"At this time, maritime operations are effectively shut down at the Port of Oakland. Maritime area operations will
resume when it is safe and secure to do so," a spokesman for the port said in a statement.
Al Jazeera's Rob Reynolds, reporting from Oakland, said the mostly peaceful events had been marred by sporadic "scuffles and fistfights" between protesters.
Reynolds said some young, black-clad protesters had tried to break windows and spray graffiti on banks and markets, while other groups tried to prevent them from doing so.
"What been really striking is the almost complete absence of police presence. We have seen very, very few policemen and there were no policemen there when demonstrators were throwing rocks and breaking windows," Reynolds said.
"It's almost like the police have withdrawn from the streets."
Local labour leaders, while generally sympathetic to the protesters, said their contracts prohibited them from proclaiming an official strike.
Craig Merrilees, a spokesman for the International Longshore and Warehouse Union, said about 40 out of 325
unionized port workers had stayed off the job.
"There was no call for a strike by the union," he said. City officials also said schools and government offices
were remaining open.
Oakland school officials said about 360 teachers did not show up for work, roughly 18 per cent of the district's teaching staff.
Oakland Mayor Jean Quan said in a statement that she supported the goals of the protest movement but the city has a duty to ensure public safety.
The protesters are part of the "Occupy" movement against corporate greed which, inspired initially by anti-Wall Street demonstrations in a New York park, has spread to other US and worldwide cities.
Activists in Oakland have been camping in a central plaza the northern Californian city since October 9.
But tensions in the city have been heightened since police used tear gas, batons and concussion grenades to clear the plaza on October 25, making 85 arrests and removing facilities set up by protesters including a kitchen, a library, medical supplies and bicycle-powered computers.
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In response, supporters marched through the streets, and were confronted by police who used numerous rounds of tear gas, pepper spray, concussion grenades and rubber bullets, according to witnesses.
"They wanted to show that they could stomp out the Occupy movement," Boots Riley, a local musician and an activist with Occupy Oakland, told Al Jazeera. "They wanted to use Oakland as an example."
In the melee, Scott Olsen, a 24-year-old former US Marine who served two tours of duty in Iraq, was hit in the forehead by a projectile, causing a skull fracture and brain swelling.
Olsen remains in a critical condition and has been placed in a medically-induced coma. Doctors say they expect him to recover fully, according to a friend who served with Olsen in Iraq.
Quan has since apologised for the harsh action, visited Olsen in the hospital, and allowed protesters to re-establish their camp.
"I can't change what happened, but I'm committed to making sure that we have an open dialogue and that we change what happens in the future," she said.
The day after the police crackdown, Occupy activists decided to call for Wednesday's general strike, calling on workers and students to stay out of work and school and spend their day protesting against what they describe as economic inequality and police brutality.