Caroline Lucas has branded Donald Trump "a racist bigot" and called on the UK Government "to stand up to bullies."
The MP and co-leader of the Green Party spoke out against the US President as demonstrators around the world took to the streets in protest over an executive order signed by Mr Trump that suspended immigration from seven Muslim-majority countries.
Thousands descended on London's Downing Street, calling for Theresa May to cancel the real-estate mogul's planned state visit.
During an interview with the BBC Ms Lucas was asked: "Aren't these the actions of someone who doesn't yet have the experience, who is learning very fast in a new job?"
She replied: "No, they are the actions of a racist bigot and we have to stand up to bullies and that is what the UK should be doing."
She added that Theresa May was so worried about being "friendless after Brexit that she will accept any kind of relationship with any kind of bigot around the world."
Ms Lucas, who attended the Downing Street rally along with several Labour and SNP politicians, said the Government response to Mr Trump's travel ban was "appallingly meek."
"On this cold, grey night we will stand together in solidarity with Muslims across the world who are bearing the brunt of Trump’s bigotry. We will condemn the British Government’s appallingly meek response to the US president’s barbarism. And we will demand something better," she wrote in an op-ed on the Huffington Post.
"History teaches us the consequences of silence at moments like these. All of us have a duty to stand up against oppression and hatred. So today I urge you to join a demonstration near your home, to sign the petition and to join me in telling our Government that we won’t accept them backing away from standing up to tyranny."
Refugees, green card and visa holders were blocked from boarding US-bound flights or detained in arrivals over the weekend as Mr Trump's ban on immigration from Iraq, Syria, Iran, Sudan, Libya, Somalia, and Yemen came into force for 90 days.
Protesters swarmed international travel terminals and took to the streets denouncing the measures, which also suspended refugee resettlement for 120 days and barred Syrian refugees indefinitely.
The UN denounced the measures as "mean-spirited" and "illegal" under international human rights law.
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