Operations at one of the busiest US ports "partially resumed" Thursday, a day after they were closed down following anti-Wall Street protests, officials said.

The Port of Oakland said staff were returning to work after the docks, the fourth biggest in the United States by cargo volume according to its website, were closed overnight as clashes flared in the California city.

"Workers in the maritime area of the port are beginning to return to their jobs and seaport operations have partially resumed, though the situation remains fluid," it said in an update.

No injuries were reported, although some protestors did briefly get into the port area, it added.

"There were no injuries, no property damage, and no major security problems from last night's demonstrations. There was a limited incursion into a private rail facility, and trespassers were escorted off peacefully," it said.

The Oakland port -- which does 59 percent of its trade with Asia -- sent staff home early on Wednesday as hundreds of protesters besieged the docks on the San Francisco Bay.

Thousands of people had rallied in downtown Oakland during the day Wednesday, before heading for the docks in the evening, forcing the port to close.

The protests were mainly peaceful until around midnight, when dozens of protesters in the city center hurled rocks and bottles, briefly occupied a vacant building and torched a barricade.