Donald Trump tore into civil rights legend John Lewis for questioning the legitimacy of the Republican billionaire's White House victory, intensifying a feud with the black congressman days before the national holiday honouring Martin Luther King Jr. and as the first African-American president prepares to leave office.

Trump tweeted on Saturday that Lewis, a Democrat, "should spend more time on fixing and helping his district, which is in horrible shape and falling apart (not to mention crime infested) rather than falsely complaining about the election results."

The incoming president added: "All talk, talk, talk — no action or results. Sad!"

Congressman John Lewis should spend more time on fixing and helping his district, which is in horrible shape and falling apart (not to...... —@realDonaldTrump

mention crime infested) rather than falsely complaining about the election results. All talk, talk, talk - no action or results. Sad! —@realDonaldTrump

Lewis, among the most revered leaders of the civil rights movement, suffered a skull fracture during the march in Selma, Ala., more than a half-century ago and has devoted his life to promoting equal rights for African-Americans.

Trump's made more comments about Lewis on Twitter Saturday evening, tweeting that ""Congressman John Lewis should finally focus on the burning and crime infested inner-cities of the U.S. I can use all the help I can get!"

Congressman John Lewis should finally focus on the burning and crime infested inner-cities of the U.S. I can use all the help I can get! —@realDonaldTrump

The weekend clash highlighted the sharp contrast between how many African-American view Trump's inauguration compared with Barack Obama's eight years ago.

It also demonstrated that no one is untouchable for scorn from a president-elect with little tolerance for public criticism. Trump has found political success even while attacking widely lauded figures, before and after the campaign — a prisoner of war, parents of a slain U.S. soldier, a beauty queen and now a civil rights icon.

Lewis, a 16-term congressman, said Friday that he would not attend Trump's swearing-in ceremony at the Capitol this coming Friday. It will mark the first time he skipped an inauguration since joining Congress in 1986.

"You know, I believe in forgiveness. I believe in trying to work with people. It will be hard. It's going to be very difficult. I don't see this president-elect as a legitimate president," Lewis said in an interview with NBC's Meet the Press set to air Sunday.

"I think the Russians participated in helping this man get elected. And they helped destroy the candidacy of Hillary Clinton," Lewis said.

Lewis' spokeswoman, Brenda Jones, declined to respond to Trump and said the lawmaker's "opinion speaks for itself."

"We as a nation do need to know whether a foreign government influenced our election," she said.

U.S. intelligence agencies have said that Russia, in a campaign ordered by President Vladimir Putin, meddled in the election to help Trump win. After spending weeks challenging that assessment, Trump finally accepted that the Russians were behind the election-year hacking of Democrats. But he also emphasized that "there was absolutely no effect on the outcome of the election, including the fact that there was no tampering whatsoever with voting machines."

Democrat Clinton received 2.9 million more votes than Trump but lost the Electoral College vote.

Defence from both sides

Lewis's Democratic colleagues quickly came to his defence Saturday.

"Let us remember that many have tried to silence Rep. John Lewis over the years. All have failed," tweeted House Minority Leader Nancy Pelosi.

The Democratic Party of Georgia called on Trump to apologize to Lewis and the people of his district.

"It is disheartening that Trump would rather sing the praises of Vladimir Putin than Georgia's own living social justice legend and civil rights icon," state party spokesman Michael Smith said.

Several of Trump's fellow Republicans also criticized the president-elect's tweets.

Michael Steele, who served as the first black chairman of the Republican National Committee until 2011, said Trump's tweets were unfortunate.

"John Lewis has a walk that very few people in this country, least of all Donald Trump, have ever walked, so you have to respect that," Steele said on MSNBC.

If Trump is looking to fix a bridge to black voters, their expectation is he "will do so in a way that shows respect for our leadership," Steele said.

Rev. Jesse Jackson also responded to the Trump attack.

"Trump does not have the moral authority to battle with John Lewis," he told CBC News. "His arms are too short to box with John Lewis."

Jackson responds to Trump's comments about Democratic congressman and civil rights activist John Lewis 1:14

Claims hard to prove

Trump won the presidency with less support from black and Hispanic voters than any president in the last 40 years, only eight per cent and 28 per cent respectively, polling data showed.

Trump's assertion that Lewis' district is "falling apart" and "crime infested" is hard to test.

Georgia's 5th Congressional District includes the Atlanta metro region, which has a large African-American population. The district is considered one of the nation's fastest growing areas, but its crime and poverty rates are higher than the national average.

The district has an 8.2 per cent unemployment rate and the median household income is about $48,000, according to the Census Bureau.

The area also covers part of the upscale Atlanta neighbourhood of Buckhead, along with the headquarters for Fortune 500 companies such Coca-Cola and Delta Air Lines, Emory University, Georgia Tech, several historically black colleges and universities and the Hartsfield-Jackson Atlanta International Airport, one of the world's busiest.