Film director Michael Moore has urged a crowd of Trump protesters in New York to regularly call their representatives in Congress.

The mayors of NYC and Minneapolis joined actors and thousands of New Yorkers, gathering outside one of Mr Trump’s buildings near Central Park for a demonstration on the eve of the presidential inauguration.

Actor Alec Baldwin, who portrays an exaggerated Trump on NBC’s Saturday Night Live, briefly did his impression for the crowd.

He told them Americans wary of Trump’s policies should become more civically involved.

The rally concluded with the crowd — estimated by police at more than 20,000 — singing the Woody Guthrie anthem, “This Land Is Your Land.”

Mike Moore addresses one of 30 rallies in Washington. Photo: AAP

Actors Robert De Niro, Sally Field and others plan to join the protest to rally cities to chart their own course on health care, climate change and other issues during the Republican’s administration.

Actor Mark Ruffalo said in a statement that communities across the country had the “power to protect people and the environment” and to unite to oppose what participants see as harmful policies Mr Trump may advance.

Meanwhile, Washington has turned into a virtual fortress ahead of the inauguration, with police ready to step in to separate protesters from Mr Trump supporters at any sign of unrest.

Actor Mark Ruffalo urging Americans to unit to oppose Trump’s “harmful policies”. Photo: Getty

Some 900,000 people, both Trump backers and opponents, were expected to flood Washington for the inauguration ceremony (early Saturday morning Australian time), according to organisers’ estimates.

Events include the swearing-in ceremony on the steps of the US Capitol and a parade to the White House along streets thronged with spectators.

The number of planned protests and rallies this year is far above what has been typical at recent presidential inaugurations, with some 30 permits granted in Washington for anti-Trump rallies and sympathy protests planned in cities from Boston to Los Angeles, and abroad in cities including London and Sydney.

In Washington, police cars lined much of Pennsylvania Avenue, the parade route, as workers unloaded crowd control fences from flatbed trucks, erected barricades and marked off pavement with tape.

Obama supporters and protestors are also gathering in Washington. Photo: AP/Matt Rourke

US Department of Homeland Security Secretary Jeh Johnson said police aimed to keep groups separate, using tactics similar to those employed during last year’s political conventions.

“The concern is some of these groups are pro-Trump, some of them are con-Trump, and they may not play well together in the same space,” Johnson said on MSNBC.