AMNA NAWAZ: Britain's Prime Minister Keir Starmer, who's been in office just a month, has promised to crack down on groups of what he called right-wing thugs who've ignited a week of racist anti-immigrant riots in towns and cities across England.

Nearly 400 people have been arrested in the past few days, and more arrests are expected.

From England, special correspondent Malcolm Brabant reports.

MALCOLM BRABANT: These are just some of the social media videos that detectives are trawling through to identify and prosecute perpetrators participating in and stoking Britain's worst riots since 2011.

After days of unrest, the crisis deteriorated over the weekend, when a mob attacked a hotel housing asylum seekers in the northern city of Rotherham.

The government minister responsible for law and order is Home Secretary Yvette Cooper.

YVETTE COOPER, British Home Secretary: It's a total disgrace, and there has to be a reckoning.

Those individuals who were involved in the disorder need to know that they will pay a price.

MALCOLM BRABANT: After forcing police to retreat, some of the mob tried to set fire to the hotel full of people of color.

Prime Minister Keir Starmer: KEIR STARMER, British Prime Minister: If you target people because of the color of their skin or their faith, then that is far right, and I'm prepared to say so.

MAN: Your tats are awesome, man.

Pure British.

Yes, mate!

MALCOLM BRABANT: The man with the Nazi swastika tattoo replied that he was a true Englishman, amid an expletive-filled rant against foreigners.

Inaccurate social media posts triggered violence across the country last week after these three girls were stabbed to death at a dance class in Southport in Northwest England; 17-year-old Axel Rudakubana, British-born son of Rwandan parents, with a history of autism, has been charged with three counts of murder and 10 of attempted murder.

The supposed motive for the riots has now pivoted to anger over the impact of mass immigration.

Keir Starmer again: KEIR STARMER: This is violence.

It is not protest.

It doesn't matter what the motivation is.

The motivation for the vast majority of people in this country is to see their streets safe.

MALCOLM BRABANT: As asylum seekers cowered inside the wrecked hotel in Rotherham, 70 miles to the south in the city of Tamworth, a similar migrant shelter came under attack from would-be arsonists.

YVETTE COOPER: There will be people who were thinking they were going on their summer holidays this week, and instead they will face a knock on the door from the police.

They will face arrest and a prison cell.

MALCOLM BRABANT: So far, there have been hundreds of arrests.

But there are warnings that the government may struggle to deliver swift justice.

CASSIA ROWLAND, British Institute for Government: The situation in the criminal justice system in England and Wales is really dire at the moment.

MALCOLM BRABANT: Cassia Rowland is a criminal justice specialist with Britain's Institute of Government.

CASSIA ROWLAND: We are out of prison capacity.

We're already releasing people early.

we've got major backlogs in the courts.

So I think it is a really difficult thing for them to do to try and fast-track that in that way.

MALCOLM BRABANT: Britain's four million-strong Muslim community is concerned about being targeted by the mobs.

In some cities, young Muslim men have turned out in force to counterprotest.

In an attempt to head of racial clashes, the government has pledged to protect the nation's 1,500 mosques.

But is it enough?

SHAFFAQ MOHAMMED, City Councillor, Sheffield, United Kingdom: If the police can't cope, then maybe you need to look at the other option of bringing the army.

That's not going to be a great look for the United Kingdom, and I don't want that to happen.

But, ultimately, we have got to maintain law and order.

You cannot have anarchy on our streets.

MALCOLM BRABANT: Shaffaq Mohammed is a former member of the European Parliament and a councillor for a district next to Rotherham.

Do you think social cohesion in Britain is breaking down?

SHAFFAQ MOHAMMED: What I don't want is for people to live parallel lives, where we never talk to each other, we never interact with each other.

And that's what fosters mistrust between different communities.

MALCOLM BRABANT: Apparently unperturbed by the government crackdown, one right wing channel on the Telegram messaging service is trying to coordinate attacks against pro-immigration lawyers and other asylum centers.

Restoring public order has become the new Labor government's most serious challenge.

At stake is Britain's future as a cohesive multicultural society.

For the "PBS News Hour," I'm Malcolm Brabant.