Thousands of demonstrators are expected in Washington and New York on Saturday to protest the killings of unarmed black men by US police and to urge Congress to protect citizens.

Organizers said the protests would be among the largest over police tactics and the killings of black males by officers in New York, Cleveland and Ferguson, Missouri.

The lack of criminal charges from grand juries in the New York and Ferguson cases have galvanized protests around the United States.

"We need more than just talk; we need legislative action that will shift things both on the books and in the streets," civil rights leader the Rev. Al Sharpton, whose National Action Alliance is heading the Washington protest, said in a statement.

Sharpton said Congress needed to pass legislation that would let federal prosecutors take over cases involving police. Local prosecutors who work with police regularly and then must investigate officers face a conflict of interest, he said.

The Washington protest will include the families of Eric Garner and Akai Gurley, who were killed by New York police; Trayvon Martin, slain by a Florida neighborhood watchman in 2012; and Michael Brown, killed by an officer in Ferguson.

The march will start at noon (1700 GMT) and block Pennsylvania Avenue between the White House and the U.S. Capitol. Protesters are expected to arrive by bus from as far away as Florida, Connecticut and Pittsburgh, according to the organizers' website.

In New York, the march was expected to draw about 44,000 people and was meant to reinvigorate protests that swelled after a grand jury declined to indict the officer who killed Garner using a choke hold, organizers said.

"It's open season on black people now," New York march co-organizer Umaara Elliott said in a statement. "So we demand that action be taken at every level of government to ensure that these racist killings by the police cease."

The march was to start at 2 p.m. (1900 GMT) at Washington Square, go to midtown Manhattan and then turn downtown to end at New York Police Department headquarters in lower Manhattan.