Donald Trump has torn into revered civil rights campaigner John Lewis for questioning the legitimacy of the Republican billionaire's White House victory.
His move intensified a feud with the black congressman days before the US national holiday honouring Martin Luther King Junior, and as the first African-American president prepares to leave office.
Trump tweeted on Saturday that Lewis "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!"
John Lewis, among the most revered leaders of the civil rights movement, suffered a skull fracture during the march in Selma, Alabama, more than half a century ago and has devoted his life to promoting equal rights for African-Americans.
The weekend clash highlighted the sharp contrast between how many African-Americans 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 dead US soldier, a beauty queen and now a civil rights campaigner.
Mr Lewis, a 16-term congressman, said on Friday that he would not attend Trump's swearing-in ceremony at the Capitol this coming Friday. It will mark the first time he has missed an inauguration since joining Congress three decades ago.
"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 on 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 his "opinion speaks for itself".
"We as a nation do need to know whether a foreign government influenced our election," she said.
US 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 emphasised 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 Hillary Clinton received 2.9 million more votes than Trump but lost the Electoral College vote.
The Democratic Party of Georgia called on Mr Trump to apologise 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.