Eric Adams, the New York mayor, said that city police officers would be ready to “respond to any and all circumstances, including large-scale protests”.

Some of Mr Trump’s supporters in the Republican Party – for whom he is the nominee for November’s presidential election – on Thursday rallied to support the former president.

Marjorie Taylor Greene, the congresswoman for Georgia who has been one of Mr Trump’s fiercest defenders on Capitol Hill, tweeted an image of an upside down US flag, which is frequently used by Right-wing Americans to signify political dissent.

The flag was waved by some of Mr Trump’s supporters when they stormed the US Capitol on January 6, 2021.

Mike Johnson, the Speaker of the House of Representatives, has declared that Donald Trump’s conviction marks a “shameful day in American history”.

Mr Johnson, who has allied himself closely with the former president, claimed Trump had been found guilty on “the testimony of a disbarred, convicted felon” - referring to Michael Cohen.

He added: “This was a purely political exercise, not a legal one... The American people see this as lawfare, and they know it is wrong—and dangerous. 

“President Trump will rightfully appeal this absurd verdict—and he WILL WIN!”

Lindsey Graham, a Trump ally and senator for South Carolina, said: “This verdict says more about the system than the allegations.”

“Congratulations, progressives,” said Utah senator Mike Lee. “You’ve just guaranteed Trump’s election.”

Some of Trump’s detractors in the Republican Party were also highly critical of the trial outcome.

Texas senator John Cornyn, who had distanced himself from the former president after the Capitol attack, said “this verdict is a disgrace, and this trial should have never happened.”

He added: “Now more than ever, we need to rally around @realdonaldtrump, take back the White House and Senate, and get this country back on track.”

Major Republican donors backed Mr Trump on Thursday. Casino billionaire Miriam Adelson and hotelier Robert Bigelow vowed to send more money to the former president and Mr Bigelow, who has already given over $9 million to an outside group supporting Mr Trump, said the criminal proceedings were a “disgrace”.

“I’m sending President Trump another $5 million as I promised him,” Mr Bigelow said.

Elon Musk, the billionaire, tweeted that the verdict was “troubling”. Mr Musk was on Wednesday linked with a possible role in a future Trump administration.

Shares of Trump Media & Technology Group, the owner of social networking site Truth Social, slumped nearly 9 per cent in after-hours trading on Thursday.

Hillary Clinton posted an image on Instagram of a mug that features a cartoon of her drinking from and the words “turns out she was right about everything”.

Democrats, who began fundraising off the news within minutes of the verdict, reacted with joy to the jury’s decision.

“Trump toadies are going to attack the jury and the court because they have a plan to dismantle our democracy and it relies on everyone believing the justice system is rigged,” posted Chris Murphy, a senator for Connecticut.

“It isn’t. Donald Trump committed a crime. He got caught. He got convicted. That’s the rule of law.”