Donald Trump has sparked fury after tearing into renowned civil rights leader John Lewis for questioning the legitimacy of the Republican billionaire's White House victory.
Mr Trump's Twitter outburst has intensified a feud with the black congressman days before the national holiday honouring Martin Luther King.
Trump tweeted that Georgia Democrat Mr 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!"
Mr Lewis, among the most revered leaders of the civil rights movement, had his skull fractured during the march in Selma, Alabama, more than 50 years ago and has since devoted his life to promoting equal rights for African-Americans.
The weekend clash highlighted the sharp contrast between how many African-Americans view Mr Trump's inauguration compared with Barack Obama's eight years ago.
It also demonstrated that no one is immune from scorn from a president-elect with little tolerance for public criticism.
Mr 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 icon.
Mr Lewis, a 16-term congressman, said on Friday that he would not attend Mr Trump's swearing-in ceremony at the Capitol on January 20 - the first time he had skipped an inauguration since joining the US Congress 30 years ago.
"You know, I believe in forgiveness. I believe in trying to work with people. It will be hard," he told NBC's Meet The Press in an interview to be shown on Sunday .
"It's going to be very difficult. I don't see this president-elect as a legitimate president."
Mr Lewis added: "I think the Russians participated in helping this man get elected. And they helped destroy the candidacy of Hillary Clinton."
His spokeswoman, Brenda Jones, declined to respond to Mr Trump and said the Mr Lewis' "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 Mr Trump win.
After spending weeks challenging that assessment, Mr 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 Mrs Clinton received 2.9 million more votes than Mr Trump but lost the Electoral College vote.
Mr Lewis' Democratic colleagues rushed to his defence on Saturday.
California's Ted Lieu said he too would skip Mr Trump's inauguration.
"For me, the personal decision not to attend the inauguration is quite simple: Do I stand with Donald Trump, or do I stand with John Lewis? I am standing with John Lewis," he said.
The Democratic Party of Georgia called on Mr Trump to apologise to Mr Lewis and the people in 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.
But Mr Trump continued to jab Mr Lewis on Saturday night, saying that the congressman "should finally focus on the burning and crime infested inner-cities of the US".
"I can use all the help I can get!" Mr Trump tweeted.
Yet the president-elect's assertion that Mr Lewis' district is "falling apart" and "crime infested" is hard to prove.
Georgia's 5th Congressional District includes the Atlanta metro region, considered one of the nation's fastest-growing areas.
Its crime and poverty rates are higher than the national average.
Crime statistics for the specific district are not measured by the government and Atlanta officials have reported a significant drop in crime in recent years, although they created a gun violence task force last year to address an increase in murders.
The district has an 8.2% unemployment rate and the median household income is about 48,000 dollars (£39,300), according to the Census Bureau.
The area covers part of the wealthy 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.
But the dispute may be helping sales of Mr Lewis' books.
His defenders have been urging Twitter followers to buy them - a strategy apparently succeeding.
By Saturday night, a bound collection of his March trilogy - graphic memoirs for young people about his civil rights activism - was number one on Amazon and a more traditional memoir, Walking With The Wind, was second.
Last autumn, the third of Mr Lewis' March books, on which he collaborated with Andrew Aydin and Nate Powell, won the National Book Award in the young people's literature category.