The Metropolitan Police said it was “ready to share resources across the country” if disorder breaks out at protests planned for Saturday.
A demonstration called by the Democratic Football Lads Alliance (DFLA) is expected to draw the largest numbers in Westminster, as Brexiteers take to the streets in Manchester, Birmingham and other cities.
The protests were organised amid a surge in anger over parliament’s moves to prevent a no-deal Brexit with a bill that was approved by the House of Lords on Friday.
Extremists using numerous far-right channels on the encrypted Telegram messaging app calling MPs who backed the bill “traitors” and “scum” while planning rallies.
One member of a chat group purporting to contain supporters of Nigel Farage’s Brexit Party threatened “Brexit or we burn the country” after a fellow user called for unrest like that seen in Hong Kong.
A member of another group used by Britain First supporters wrote: “I predict a riot, then serious civil unrest until the 31st. Protect your loved ones and property, Britain is about to go in to biblical battle.”
“It’s time to f*** s*** up,” another post read. “Let’s f***ing smash um! Britain take to the streets!”
Another person wrote: “It needs to kick of Saturday big time.”
In the same chat, which includes racist and Islamophobic memes, a user called for “some good old civil unrest”.
In a different group for “patriots”, people were sharing photos of smoke grenades they had bought online to take to the protests.
The Metropolitan Police said it was prepared for the demonstrations and the possibility of further protests in the run-up to Brexit.
“To date, protests have been largely peaceful and we have no intelligence at this time to suggest that will change,” a spokesperson added.
“Our officers are well trained to maintain public order and stand ready to share resources across the country if any disorder breaks out. We will not hesitate to take necessary action against anyone who deliberately chooses to act outside the law.”
Senior police officers have been calling for politicians and other political figures to avoid worsening tensions with inflammatory language.
Martin Hewitt, chair of the National Police Chiefs’ Council, issued a warning over the “incredibly febrile atmosphere”.
“If you’re in a position where you know you’re going to be listened to, you need to be very careful about the language you are using so it doesn’t end up with consequences that weren’t intended,” he urged.
Last month, the head of UK counterterror policing said all public figures should be “bringing society together” as Brexit worsens polarisation.
“We should be really careful about what we say, we should be balanced, because there is no doubt in my mind that it has an effect,” assistant commissioner Neil Basu previously told The Independent.
Speaking in the House of Commons on Wednesday, Mr Johnson said the comments had been part of a “strong liberal defence of everybody’s right to wear whatever they want in this country”, and refused to apologise.
In the same session, he characterised a bill proposed by opposition parties to prevent a no-deal Brexit as a move against the “democratic will of the people of the UK”.
Mr Johnson described the proposed law as a “surrender bill” eight times during the session, and the term was then repeated by extremists.
Experts said the prime minister and some other pro-Brexit politicians were “using the language of the far right” and playing into extremist narratives.
Chloe Colliver, who leads the digital research unit at the Institute for Strategic Dialogue think-tank, said that by using the phrase “surrender bill” and positioning himself as enacting the “will of the people”, Mr Johnson was “calling successfully to a nationalist interpretation of the Brexit debate”.
“It seems very purposeful to me and it really harks back to the Second World War nostalgia in this debate, which plays powerfully to the far right and nationalist groups,” she told The Independent.
“They don’t understand the mechanisms and decisions being made on their behalf, and that’s a ripe opportunity for these groups to step in with a simple narrative that pits them against everyone else.”
Mr Johnson is currently receiving substantial support from key far-right figures, including former Britain First leader Jayda Fransen, who praised him for “purging the traitors” in his party and called on others to fall behind him.
The operators of Tommy Robinson’s official Telegram channel called on people to “back Boris”, while supporters shared memes depicting him as Winston Churchill.
Ms Colliver warned that if the far right feel Mr Johnson is “in their corner”, it may reduce unrest in the short term but make them feel that their ideals have mainstream support.
She said the political stalemate over Brexit was undermining faith in democratic processes, and being “used to channel violent undemocratic objectives to a newly broad and receptive audience in the UK.”
Dr Joe Mulhall, senior researcher at Hope Not Hate, said far-right groups had seized upon Brexit as a nationalist cause and used it to gain mainstream support.
He said the global extreme right-wing had celebrated the result of the 2016 referendum, but then worked the following delays into a “narrative of betrayal” by “elites”.
“Their sense of betrayal could well spill out into anger and violence,” Dr Mulhall said. “They want no deal but if things go terribly it will be blamed on politicians, or the corrupt elite or global powers. Whatever the outcome is, they will be outraged.”
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