US civil rights activists vowed on Saturday to defend hard-fought gains in voting rights and criminal justice during the presidency of Donald Trump, kicking off a week of protests ahead of the Republican's inauguration.

About 2,000 mostly black protesters ignored steady rain to march and rally near Washington's Martin Luther King Jr Memorial, as speakers urged them to fight for minority rights and President Barack Obama's signature healthcare law, which Trump has vowed to dismantle.

The Rev Al Sharpton, the rally's organiser and a veteran civil rights leader, said Democrats in Congress needed to be sent a simple message: "Get some backbone."

"We march in the driving rain because we want the nation to understand that what has been fought for and gained, that you're going to need more than one election to turn it around," he said.

The rally drew fewer people than organisers had initially expected, but Sharpton said afterwards he was satisfied with the turnout, given the rain and temperatures hovering just above freezing.

"I really didn't think we'd get those kind of numbers," he said in a telephone interview.

Trump, a New York real estate developer, won with a populist platform that included promises to build a wall along the Mexican border, restrict immigration from Muslim countries and dismantle Obamacare.

His choice of Senator Jeff Sessions, an Alabama Republican, to become attorney general has raised concern among many on the left that Trump could weaken voting rights for minorities and roll back criminal justice reforms.

"We will march until hell freezes over, and when it does, we will march on the ice," said Cornell William Brooks, president and chief executive of the National Association for the Advancement of Coloured People.

The rally also included the Hispanic group La Raza, politicians, relatives of African-Americans slain by police, the National Urban League, Planned Parenthood and the Human Rights Campaign, an advocacy group for lesbian, gay, bisexual and transgender civil rights.

The rally came hours after Trump blasted US Representative John Lewis after the Georgia Democrat and civil rights campaigner said Russia's alleged hacking aimed at helping Trump put his legitimacy into question.

Trump replied on Twitter that Lewis should focus instead on his Atlanta district. "All talk, talk, talk ─ no action or results! Sad!," he wrote.

About 30 groups, almost all of them anti-Trump, have gotten permits to protest before, during and after the inauguration. Thousands of demonstrators have vowed to shut down the inauguration.

Washington police and the US Secret Service plan to have some 3,000 extra officers and an additional 5,000 National Guard troops on hand for security.

By far the biggest event will be the Women's March on Washington the day after the inauguration, which organisers say could draw 200,000 people.