US President Donald Trump has called the congressional impeachment investigation that may seek to remove him from office "a lynching".
Mr Trump condemned the inquiry in a tweet as "without due process or fairness or any legal rights".
Democrats may seek to impeach the Republican president by Christmas.
A racially loaded term in the US, lynching refers to historic extrajudicial executions by white mobs mainly against African Americans.
Skip Twitter post by @realDonaldTrump
So some day, if a Democrat becomes President and the Republicans win the House, even by a tiny margin, they can impeach the President, without due process or fairness or any legal rights. All Republicans must remember what they are witnessing here – a lynching. But we will WIN!
— Donald J. Trump (@realDonaldTrump) October 22, 2019
End of Twitter post by @realDonaldTrump
White House spokesman Hogan Gidley told reporters on Tuesday that Mr Trump did not mean to be racially insensitive.
"The president has used many words, all types of language, to talk about the way the media has treated him," the deputy press secretary said.
He added: "The president wasn't trying to compare himself to the horrific history in this country at all."
The impeachment inquiry is examining whether the Republican president abused his office by improperly pressuring Ukraine to launch an investigation into former US Vice-President Joe Biden, a leading candidate for the Democratic 2020 presidential nomination.
Mr Trump denies holding up US military aid to Ukraine so they would investigate Mr Biden's son, who worked for a Ukrainian gas company.
What's the reaction?
Mr Trump has repeatedly called the impeachment inquiry, as well as an earlier investigation into foreign meddling in the 2016 election, a "witch hunt".
But his likening of a process enshrined in the US constitution to racist killings provoked outrage among African-American lawmakers.
Jim Clyburn, a South Carolina Democrat, told CNN: "That is one word that no president ought to apply to himself."
"I've studied presidential history quite a bit and I don't know if we've ever seen anything quite like this."
Bobby Rush, an Illinois Democrat, tweeted to Mr Trump: "What the hell is wrong with you?
"Do you know how many people who look like me have been lynched, since the inception of this country, by people who look like you."
He called on Mr Trump to delete the tweet.
Hakeem Jeffries, a New York Democrat, said: "The president should not compare a constitutionally mandated impeachment inquiry to such a dangerous and dark chapter of American history."
Republican lawmakers mostly defended Mr Trump, but not necessarily his choice of language.
Jim Jordan, of Ohio, said the comment showed that "the president's frustrated".
Senator Lindsey Graham said: "This is a lynching in every sense."
Tim Scott, the only black Republican US senator, said: "There's no question that the impeachment process is the closest thing to a political death row trial, so I get his absolute rejection of the process.
"I wouldn't use the word lynching."
House Republican leader Kevin McCarthy told reporters: "That's not the language I would use."
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