The three videos Trump shared were titled "Muslim migrant beats up Dutch boy on crutches!," "Muslim destroys a statue of Virgin Mary!" and "Islamist mob pushes teenage boy off roof and beats him to death!"
The videos provide no context. The Netherlands Embassy tweeted to Trump that the video about the "Muslim migrant" had been mischaracterized: "Facts do matter. The perpetrator of the violent act in this video was born and raised in the Netherlands. He received and completed his sentence under Dutch law." The embassy did not reveal his religion.
White House press secretary Sarah Huckabee Sanders defended Trump's post as evidence he wants to "promote strong borders and strong national security."
"Whether it's a real video, the threat is real, and that is what the president is talking about," Sanders told reporters.
Trump retweeted an item from Jayda Fransen, 31, deputy leader of Britain First, who in an interview with The Washington Post expressed gratitude for what she said was Trump's endorsement of her and her group.
"The British establishment no longer supports free speech, but the president of the United States, Donald Trump, clearly does, and that's why he tweeted, as a public display of support for Britain First and its deputy leader," she said.
Fransen faces a charge of hate speech for remarks she made about Muslims at a recent rally in Belfast and a charge of "religious harassment" for a social-media campaign she waged against several Muslim men convicted in a rape case.
Criticism of the president's retweets came thick and fast in Britain, drawing in Prime Minister Theresa May, whose office said Trump was "wrong" to promote the videos.
May's office condemned Britain First for its use of "hateful narratives which peddle lies and stoke tensions."
The statement continued, "The British people overwhelmingly reject the prejudiced rhetoric of the far-right, which is the antithesis of the values that this country represents — decency, tolerance and respect. It is wrong for the President to have done this."
Trump did not apologize or explain. Instead, Wednesday night he tweeted a response directly to the British prime minister: "@Theresa_May, don't focus on me, focus on the destructive Radical Islamic Terrorism that is taking place within the United Kingdom. We are doing just fine!"
British leaders across the political spectrum said they were amazed and appalled by Trump's tweets endorsing a group that usually draws just a few dozen supporters to its rallies. Some said Trump was trying to legitimize the far right in Britain, while others were so flabbergasted that that they wondered whether he was perhaps either naive or ignorant.
"Britain First is an appalling organization," Martin Callanan, a Conservative Party politician and government minister, told the BBC.
Referring to Trump, Callanan said: "I can only assume he has made a mistake and that he didn't realize who Britain First were."
Asked about the president's response to May's statement, a deputy White House press secretary, Raj Shah, told reporters: "The president has the greatest respect for the British people and for Prime Minister May."
When a reporter asked why the president was retweeting posts from a far-right, anti-Muslim party condemned by the British leadership, Shah said: "We're going to be focusing on the issues that are being raised, which is safety and security for the American people." He noted that Trump had supported "extreme vetting" of foreigners traveling to the United States and other measures to crack down on possible entry by terrorists.
None of the videos appeared to deal with people traveling to the United States.
Britain First leaders were ecstatic about the recognition from the U.S. president.
"DONALD TRUMP HIMSELF HAS RETWEETED THESE VIDEOS AND HAS AROUND 44 MILLION FOLLOWERS! GOD BLESS YOU TRUMP!" Fransen tweeted.
Britain First was founded in 2011 by Jim Dowson, an antiabortion campaigner, and Paul Golding, a former local-government councilor for the British National Party.
Britain First soon became known for its "Christian patrols" and for driving around in paramilitary-style vehicles and wearing uniforms.
Its members are notorious for targeting mosques and majority-Muslim areas and then producing short, selectively edited videos of their provocative tactics.
Over the past spring and summer, for instance, a few dozen activists with Britain First marched in front of the East London Mosque, waving the Union Jack and carrying white crosses.
In one exchange, a counterprotester shouts, "What you're doing is disgusting!" And someone shouts back, "This is still a British Christian area, and this is our country."
The videos show scuffles, kicking, cursing and egg-throwing as police struggle to keep the two sides apart.
In a video clip from June, Golding is shown in front of the mosque saying, "This used to be our area," and vowing, "It will be our area again."
He narrates that his group walked past the mosque and that "we were very quickly surrounded by an ever-increasing mob of Muslims and white liberals screaming abuse."
"We had things thrown at us, we had people spit at us, and this was for the heinous crime of standing on a British pavement and filming. That's all we did. . . . Whose country is this?"
In August 2016, the group's leaders were banned from entering all mosques in England and Wales and from encouraging their followers to do so.
Later that year, Golding was jailed for violating the ban.
On his release from prison, he put out a chilling video in which he rants against the establishment, liberals, the media and "foreign infidels."
In the video, Golding threatens to "confront and oppose every traitor in this county." As he condemns "traitors," the short video shows images of the British prime minister wearing a headscarf and meeting young Muslim girls.
Golding said he was jailed because he had the courage and conviction to confront the hard-line Muslim cleric Ali Hammuda, the imam at a mosque in Cardiff that is home to several Britons who traveled to join the Islamic State militant group in Syria.
Last year, a court in Luton found Fransen guilty of verbally abusing a Muslim woman.
"They have been quite provocative over the years," said Dilowar Khan, director of finance and engagement at the East London Mosque. "They come to provoke the local youths and intimidate them. They get them to react, take video and then post it on their website and say, 'Look how nasty Muslims are.' They are a blatantly anti-Muslim group."
Once, he said, members of Britain First entered the mosque with their shoes on, making sure to walk over prayer rugs, and then delivered a Bible to the receptionist. On another occasion, he said, supporters of the group drank alcohol outside in hopes of triggering a response. On yet another occasion, they reportedly blocked the mosque's entrance with a large cross.
"It's not appropriate for any politicians to show direct or indirect support for groups that are blatantly anti-Muslim and trying to divide communities," Khan said.
Even Paul Joseph Watson of the far-right, conspiracy-minded Infowars tweeted that giving Britain First a megaphone is not a good look for Trump.
"Yeah, someone might want to tell whoever is running Trump's Twitter account this morning that retweeting Britain First is not great optics," Watson wrote.
Nick Ryan, a spokesman for Hope Not Hate, an anti-extremist research organization, said it was astonishing that the U.S. president would knowingly retweet the Britain First posts.
"A politician would have to be blind not to understand that this is a particularly nasty far-right organization that is in trouble with the law, electoral authorities, and reviled by 99 percent of the population," Ryan said.
Although Britain First draws no more than a few hundred people to its rallies, it has a massive following online — its Facebook page has nearly 2 million likes. But Hope Not Hate has questioned the legitimacy of the online support.
"We think they may have bought a proportion of their followers," Ryan said.
In June 2016, a Labour Party member of Parliament, Jo Cox, was shot and stabbed to death by an assailant alleged to have shouted "Britain first!" Leaders of the Britain First group said there were no ties between the attacker and their organization. The assailant had links to neo-Nazi groups.
On Wednesday morning, Cox's widower, Brendan Cox, tweeted, "Trump has legitimized the far right in his own country, now he's trying to do it in ours. Spreading hatred has consequences & the President should be ashamed of himself."
Britain First has tried its hand at electoral politics but has failed to get any candidate into office.
Griff Witte in Berlin contributed to this report.