Unrest related to misinformation about a mass stabbing that killed three young girls last week spread to multiple towns and cities on Saturday as anti-immigration demonstrators clashed with police.

The violence is posing a major test for Prime Minister Keir Starmer, who was elected only a month ago after leading Labour to a landslide win over the Conservatives.

About 90 people were arrested after skirmishes broke out at far-right rallies in numerous places 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 shops, while demonstrators shouted anti-Islamic slurs as they clashed with counter-protesters.

The violence is the worst England has seen since the summer of 2011, when widespread rioting took place following the police killing of a mixed-race man in north London.”We’ve had riots and clashes of this nature, but they have been pocketed in particular areas of the country. We’re now seeing it flooding across major cities and towns,” said Tiffany Lynch of the Police Federation of England and Wales.The government said the police have “all the resources they need” to deal with the disorder as officers warned of more demonstrations on Sunday, leading to fears the unrest could spread further again.

Forces have drafted in extra officers while justice minister Shabana Mahmood has insisted “the whole justice system is ready to deliver convictions as quickly as possible”.

The government’s policing minister, Diana Johnson, told BBC News Sunday that the rioting would “not be tolerated”, vowing “penalties and consequences” for the disorder.

Saturday’s 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 rallies have been advertised on far-right social media channels under the banner “Enough is enough”.

They have seen protesters waving English and British flags while chanting slogans like “Stop the boats” — a reference to irregular migrants travelling to Britain from France.

Anti-fascist demonstrators held counter-rallies in many cities, including Leeds where they shouted, “Nazi scum off our streets”, as the far-right protesters chanted, “You’re not English any more”.

Not all of Saturday’s estimated 30 gatherings turned violent and some participants claimed to have legitimate grievances.

“People are fed up with being told you should be ashamed if you’re white and working class but I’m proud white working class,” 41-year-old Karina, who did not give her surname, told AFP in Nottingham.

Commentators have suggested that the demonstrators, spurred on by online influencers, may feel emboldened by the political ascendancy of anti-immigration elements in British politics.

At last month’s election, the Reform UK party captured 14 percent of the vote — one of the largest vote shares for a far-right British party.

Starmer has accused “thugs” of “hijacking” the nation’s grief to “sow hatred” and has announced new measures to allow the sharing of intelligence, wider deployment of facial-recognition technology and criminal behaviour orders to restrict troublemakers from travelling.

Policing minister Johnson said the government will do “whatever it takes” to ensure people are brought to justice, including the possibility of courts sitting overnight as they did during the riots in 2011.