A mass protest that forced the temporary shutdown of one of the busiest ports in the U.S. erupted in violence, pitting demonstrators against police in riot gear.

Police in the western Californian city of Oakland moved in on the protesters early Thursday, firing tear gas and making arrests, after dozens of protesters broke into an empty building. Officials also accused the protesters – demonstrating as part of the larger Occupy Wall Street movement – of shattering windows and setting fires.

The Associated Press quoted Oakland police as saying some of the protesters were also throwing Molotov cocktails.

Police officials said dozens of protesters were arrested, but did not give an exact number. Several protesters were taken to the hospital with injuries. Video shows one protester falling to the ground after being hit with a projectile as he ran from police.

The violence erupted following protests Wednesday that drew as many as 7,000 people and forced the temporary closure of the Port of Oakland.

The port is the fifth largest in the U.S. and plays a key role in sending produce and electronics to Asia.

Port officials said late Wednesday they had suspended maritime operations and sent home main office employees in order “to ensure their safety” and the facilitate the smooth flow of traffic in the area. They said operations will resume “when it is safe to do so.”

Before marching on the port, most of the protesters had been massing in a downtown intersection where demonstrator and Iraq war veteran Scott Olsen was seriously injured last week during a clash between protesters and police.

Protesters say Olsen, who is now in fair condition, suffered a fractured skull after being struck in the head by a projectile launched by the police. Oakland police have opened an investigation.

The incident has further galvanized the Occupy Wall Street movement, which has sprung up in public parks and squares in major cities across the United States and around the globe.

Demonstrators have numerous demands, including ending U.S. foreign military action, raising taxes on the wealthy, and more government social spending.

Protesters have said they hope the movement will last until at least the next U.S. presidential and congressional elections in November 2012.