The UK government warned Sunday that violent disorder would "not be tolerated" after police made dozens of arrests at far-right rallies following clashes between protesters and officers.
Unrest linked to disinformation about a mass stabbing that killed three young girls last week spread to multiple towns and cities Saturday as anti-immigration demonstrators faced off against counter-protesters.
About 90 people were arrested after skirmishes broke out in numerous English cities, including Liverpool, Manchester, Bristol, Blackpool and Hull, as well as Belfast in Northern Ireland.
In some instances, rioters threw bricks, bottles and flares at police -- injuring several officers -- looted and burnt shots, while demonstrators were also heard shouting anti-Islamic slurs.
The government's policing minister, Diana Johnson, told BBC News that the rioting would "not be tolerated", and that there would be "penalties and consequences" for the disorder.
The skirmishes marked the fourth day of unrest in several towns and cities following Monday's frenzied knife attack in Southport, near Liverpool on England's northwest coast.
They were fuelled by false rumours on social media about the background of British-born 17-year-old suspect Axel Rudakubana, charged with several counts of murder and attempted murder over the attack at a Taylor Swift-themed dance party.
Rudakubana is accused of killing Bebe King, six, Elsie Dot Stancombe, seven, and Alice Dasilva Aguiar, nine, and injuring another 10 people.
Police have blamed the violence on supporters and associated organisations of the English Defence League, an anti-Islam organisation founded 15 years ago whose supporters have been linked to football hooliganism.
Agitators have targeted mosques in Southport and in the northeastern English city of Sunderland, leading to hundreds of Islamic centres to bolster security amid fears for its worshippers' safety.
The violence is the biggest test yet of Prime Minister Keir Starmer's short premiership. He came to power early last month after leading Labour to a landslide win over the Conservatives.
He has accused "thugs" of "hijacking" the nation's grief to "sow hatred" and pledged that anyone carrying out violent acts would "face the full force of the law".
After talks with senior ministers on Saturday he "reiterated that the government backs the police to take all necessary action to keep our streets safe", said a statement from his office.
Police have warned of more demonstrations on Sunday.
