More than 200 people were arrested across the United States on Wednesday as protesters marking the fifth anniversary of the US invasion of Iraq obstructed downtown traffic and tried to block access to government offices.

There were 32 arrests in Washington after demonstrators attempted to block entrances to the Internal Revenue Service, while 30 others were arrested outside a congressional office building, police said.

Protesters had hoped to shut down the IRS, the US tax collection agency, to highlight the cost of the war. Police cleared the building's entrances within an hour.

In San Francisco, long a center of anti-Iraq war sentiment, police arrested more than 100 people who protested through the day along Market Street in the central business district, a spokesman said.

Sergeant Steve Maninna said officers had arrested 143 people on charges including trespassing, resisting arrest and obstructing traffic.