The prime minister has vowed to do "whatever it takes to bring these thugs to justice" as he addressed the nation amid rioting in UK streets.
"I guarantee you will regret taking part in this disorder either directly or those whipping up this disorder online," Sir Keir Starmer said.
UK rioting - follow latest
Referring to an attack on a hotel in Rotherham currently under way, he said: "There is no justification for taking this action.
"People in this country have a right to be safe and yet we have seen Muslim communities targeted and attacks on mosques."
The Home Office on Sunday announced mosques will be offered greater protection with new emergency security.
It said mosques at risk of violent disorder can be provided with additional security personnel as part of a rapid security response to boost the work being done by local police.
Violent scenes have broken out across the country following false reports that the suspect in the death of three young girls in a knife attack at a Taylor Swift-themed dance class was an asylum seeker.
Sir Keir said he would not "shy away from calling it what it is" and branded the violence "far-right thuggery".
"To those who feel targeted because of the colour of your skin or your faith I know how frightening this must be," he said.
"I want you to know this violent mob do not represent this country and we will bring them to justice."
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