Theresa May has come under renewed pressure to toughen her stance on the controversial tycoon after he closed US borders to a host of Muslim-majority nations.
In Birmingham, protesters gathered in Victoria Square, with some chanting 'Donald Trump has lost the plot: Muslims welcome, Trump is not."
In London, thousands of activists gathered opposite the gates of Downing Street, turning Whitehall into a sea of placards.
Among the list of high-profile speakers advertised were former Labour leader Ed Miliband and pop star Lily Allen.
Shadow attorney general Shami Chakrabarti told the crowd: "It is in sadness and solidarity that we gather here this evening.
"I also hope, friends, that we stand here in solidarity with all the world's women who the president has insulted and all the desperate refugees that he would spurn."
Similar protests were planned in cities including Edinburgh, Cardiff, Manchester and Birmingham.
The action comes after a petition calling for the US president's UK state visit to be axed surged to more than a million signatures after his executive order.
Earlier on Monday, Foreign Secretary Boris Johnson sought to restore calm by telling Parliament the ban would make "no difference" to British passport holders.
Chants including "Donald Trump has got to go" rippled up the swelling crowds on Whitehall.
Traffic ground to a halt as the protesters spilled into the road.
Addressing the crowd, one Muslim refugee said: "America is a beacon of democracy and peace, but Trump's action will tear the fabric of American society.
"We call on all peace-loving individuals to stand firm and oppose all racist and xenophobic action."
Metres away from the door of Number 10, chants of "shame on May" began to echo throughout the crowd.
Downing Street earlier stood firm on the decision to extend an invitation to Mr Trump, saying: "We look forward to hosting the president later this year."
The SNP's Westminster leader Angus Robertson fired up the thousands of activists in London with a chant of "no state visit".
He added: "Friends, first they came for the Muslims, but we say something and we say no state visit for Donald Trump."
Left-wing activist Owen Jones, who helped organise the protest in London, said: "Today we say with defiance, with determination, we will not stay silent."