Police have arrested dozens of people during a raid of an Occupy San Francisco encampment outside the Federal Reserve building.

Albie Esparza, a San Francisco police officer, told the Associated Press news agency that around 4:00am local time officers arrested about 55 people for illegal lodging.

No officers or demonstrators were injured, but Esparza says some officers were spit on and one officer was pushed by demonstrators during the arrests.

Esparza says that before police moved in on the encampment, demonstrators had been warned on an hourly basis over a 24-hour period that they were subject to arrest.

The arrests on Sunday come after at least 85 people were arrested on Wednesday when police cleared a separate Occupy encampment in nearby Justin Herman Plaza.