US President Donald Trump branded African-American civil rights activist Al Sharpton a “con man” on Monday, sparking further charges of racism following his recent tirades against prominent black lawmakers.

The attack followed a weekend during which Trump drew fire for targeting Elijah Cummings, a Democratic congressman of color and high-profile Trump critic whose district covers much of the majority black city of Baltimore.

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Trump's dispute with Sharpton appeared to stem from a tweet by the civil rights leader late Sunday, stating that he was headed to Baltimore.

“Al is a con man, a troublemaker, always looking for a score. Just doing his thing,” Trump tweeted early Monday, adding that Sharpton “hates whites and cops!” Sharpton shot back, telling reporters in Baltimore that Trump “has a particular venom for blacks and people of color”.

"He can say what he wants, call me a troublemaker. Yes, I make trouble for bigots," Sharpton said.

The episode began when Trump called Cummings's district a “rat and rodent infested mess” where no one would choose to live.

The controversy comes less than two weeks after the House of Representatives condemned Trump for “racist” comments targeting four first-term Democratic congresswomen, known as the “Squad,” who are from ethnic minorities.

“If the Democrats are going to defend the Radical Left 'Squad' and King Elijah's Baltimore fail, it will be a long road to 2020,” Trump tweeted on Monday.

“So tired of listening to the same old Bull...Next, Reverend Al will show up to complain and protest. Nothing will get done for the people in need. Sad!” Trump wrote.

A historic port city of 600,000 people, Baltimore presents a mixed picture, with both handsome and affluent neighborhoods and poverty-stricken districts.

It has one of the country's highest murder rates.

'Bigoted and racist'

Trump's weekend diatribe ignited a storm of criticism, particularly from Democrats running 2020 presidential campaigns.

Presidential candidate Bernie Sanders reiterated his assertion that Trump is a racist telling CNN Sunday, “That is a disgrace and that is why we're going to defeat this president.”

Cory Booker wrote that “this is painful. This is a moral, defining moment in America” while Kamala Harris, who like Booker is black, said she was “proud” to have her campaign headquarters in Cummings's district and called Trump's attack “disgraceful”.

Trump's remarks are seen as a calculated but risky appeal, both to the disgruntled white, blue-collar base that helped get him elected in 2016, and to other whites who haven't decided whom to support in next year's elections.

After Trump's tweets attacking the Squad, his approval among Republicans rose five points to 72 per cent in a Reuters-Ipsos poll.

On Monday, Sharpton accused Trump of attacking Cummings and the people of Baltimore “in the most bigoted and racist way”.

“He attacks everybody. I know Donald Trump. He is not mature enough to take criticism. He can't help it, he is like a child — somebody says something he reacts.”

Baltimore, as well, has stood up to Trump's remarks, which Mayor Bernard “Jack” Young dubbed “completely unacceptable”.