The city repeatedly warned protesters to move from the public plaza at the foot of Market Street in recent weeks and tried unsuccessfully to negotiate a move to another location.

The camp of several hundred protesters had been kept clean and relatively orderly but was drawing increasing complaints from businesses in the area.

The San Francisco camp was the last large Occupy encampment standing after police last week dismantled tent cities in Los Angeles and Philadelphia.

The police action appeared to take protesters by surprise.

Several hundred officers surrounded the camp around 2 a.m. and, after warning people to leave, began removing tents.

More than 50 protesters were arrested and two officers assaulted, police said. Some protesters returned to the area and were reportedly planning to set up camps in other places.

Authorities in many US cities, often citing health and safety conditions, have dismantled protest camps that sprang from the original Occupy movement in New York against economic inequality and excesses of the US financial system.