Thousands of people have joined marches across Britain in protest at US President Donald Trump's travel ban on seven Muslim-majority countries.
MPs are also holding an emergency debate on Mr Trump's hastily implemented executive order.
In London, Whitehall was turned into a sea of placards as thousands gathered opposite the gates of Downing Street.
High-profile speakers such as former Labour leader Ed Miliband and pop star Lily Allen are expected to address the demonstration, organised by author and Guardian columnist Owen Jones.
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."
Mr Jones said: "Today we say with defiance, with determination, we will not stay silent."
Similar protests were planned in cities across the UK, including Edinburgh, Cardiff, Leeds, Manchester and Birmingham.
The action follows a petition calling for the US President's UK state visit to be cancelled 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" and "shame on May" rippled up the swelling crowds on Whitehall.
Earlier, Downing Street 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."
Shadow home secretary Diane Abbott told the crowd she had come on behalf of Labour leader Jeremy Corbyn.
She said: "Donald Trump has been president for only a few days, and look at what he is doing.
"We need to resist the Islamophobia and scapegoating of Muslims, we have got to resist it whether it is in the United States or here in the UK."
In Scotland, demonstrations began in the early evening.
The largest protests were in Glasgow and Edinburgh, where crowds swelled from around 5pm, and protests were also held in Aberdeen and Dundee.
Placards and banners were held aloft in Glasgow as a crowd of around 500 people chanted "hope not fear, refugees are welcome here".
The protests were organised by a variety of groups, including Stand up to Racism, and refugees whose countries had been singled out by Mr Trump.
In Edinburgh, large crowds marched from the North Bridge to the US Consulate in Regent Terrace.
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