Their criticisms come awkwardly just two days after UK Prime Minister Theresa May officially met with Trump, the first foreign leader to do so, touting the two countries' "special relationship."
Trump on Friday signed an executive order banning citizens from Syria, Iraq, Iran, Yemen, Libya, Somalia and Sudan from entering the United States for the next 90 days and suspending the admission of all refugees for 120 days.
On Friday, Trump and May celebrated their countries' "special relationship" as they walked through the White House colonnade outside the Oval Office, trading grins and sideways glances, and buttoning up agreements on NATO and trade in front of reporters. Then, barely 24 hours later, May was disagreeing with decisions her new ally has taken in her absence.
While she flew on to meet Turkey's President, then home to Downing Street, Trump was preparing the executive order banning travel from seven Muslim majority nations.
British Foreign Secretary Boris Johnson described Trump's ban as "divisive and wrong," while London Mayor Sadiq Khan said the move was "shameful and cruel."
But Prime Minister May refused to condemn the ban. Under pressure from British MPs, she later said the government does "not agree" with the executive order.
"Immigration policy in the United States is a matter for the government of the United States, just the same as immigration policy for this country should be set by our government," a spokesman for May said. "But we do not agree with this kind of approach and it is not one we will be taking."
May also ordered Johnson and Home Secretary Amber Rudd to contact their US counterparts "to protect the rights of British nationals," the Prime Minister's office said.
When she had hoped to reap the benefits of rushing to America to shore up support ahead of a bruising Brexit battle, she's being slammed to the ropes by the United Kingdom's main opposition party.
Labour leader Jeremy Corbyn is already trying to undo her White House gains and slap a travel ban on Trump, preventing his much vaunted reciprocal visit to meet the Queen.
In two tweets, Corbyn said Trump "should not be welcomed to Britain while he abuses our shared values with shameful #MuslimBan & attacks on refugees & women." He told his supporters to sign a petition, and they did, in rapidly climbing numbers.
Khan criticized Trump, saying he could not turn his back on the current global crisis.
"While every country has the right to set its own immigration policies, this new policy flies in the face of the values of freedom and tolerance that the USA was built upon," said Khan, who is a Muslim and has spoken against Trump's immigration policies before.
"As a nation that, like the USA, values tolerance, diversity and freedom, we cannot just shrug our shoulders and say: 'It's not our problem,'" he said.
Mo Farah says ban 'deeply troubling'
British Olympian runner Mo Farah, who is originally from Somalia
, wrote on Facebook that Trump's ban came from "a place of ignorance and prejudice."
"On 1st January this year, Her Majesty The Queen made me a Knight of the Realm. On 27th January, President Donald Trump seems to have made me an alien," he wrote.
Farah is a British citizen who has lived in the United States for the past six years.
"Now, me and many others like me are being told that we may not be welcome. It's deeply troubling that I will have to tell my children ... that Daddy might not be able to come home -- to explain why the President has introduced a policy that comes from a place of ignorance and prejudice," he wrote.
Farah said he was welcomed to Britain at age 8 and was given the chance to succeed and realize his dreams.
"I have been proud to represent my country, win medals for the British people and receive the greatest honor of a knighthood. My story is an example of what can happen when you follow polices of compassion and understanding, not hate and isolation."
Europe's far-right cheers
But Britain's far-right leader, Nigel Farage, welcomed Trump's executive order.
"(Trump) was elected to get tough. He was elected to say he would do everything in his power to protect America from infiltration by ISIS terrorists. There are seven countries on that list. He is entitled to do this. He was voted in on this," Farage told the BBC.
Other far-right leaders in Europe also say they agree with the ban.
"No more immigration from any Islamic country is exactly what we need," said Geert Wilders, founder and leader of the far-right Party for Freedom, on Twitter.
"Also in The Netherlands Islam and freedom are incompatible."
Merkel calls Trump
But leaders of Europe's most powerful states stood against the ban, including German Chancellor Angela Merkel, who called Trump on Saturday to explain the US' obligations under the Geneva Convention on refugees, Merkel's spokesman Steffen Seibert said.
"The German Chancellor deeply regrets the US President's decision to ban refugees and immigration of certain countries from entering the United States. She is convinced that the necessary fight against terrorism does not justify to place people of certain nationalities or beliefs under general suspicion," Seibert said.
Merkel has herself come under scrutiny in Germany for her open-door refugee policy. In 2015, the country took in more than a million Syrian refugees. Her policy has been criticized after several terror attacks in Germany since the mass intake.
French Foreign Minister Jean-Marc Ayrault said that welcoming of refugees was "a duty of solidarity."
"Terrorism doesn't have a nationality; discrimination is not an answer," he said on Twitter.
Closer to home, Canadian Prime Minister Justin Trudeau defended the importance of welcoming refugees, without explicitly referring to Trump's executive order.
He said that those fleeing persecution, terror and war were welcome in Canada.
In Iran, where leaders are trying to figure out Trump's view on them, the reaction was swift. You don't want us, then we don't want you. It issued a reciprocal 90-day travel ban for Americans wishing to visit Iran.
Across the border in Iraq, Iranian backed Shia militias fighting ISIS in cooperation with US forces called for their powerful American battlefield ally to be thrown out of the country. The Iraqi government stayed out of that fight but the message is not lost on it: A new era has arrived at the White House.