British Prime Minister Keir Starmer has reassured communities that sufficient police would be on the streets to handle anti-Muslim and anti-immigrant riots in the coming days as messages online warned of more violent protests on Wednesday.

This follows violent riots that erupted across several towns and cities stemming from false messaging on social media, wrongly suggesting that the murder of three girls at a Taylor Swift-themed event in Southport was the doing of "an Islamist migrant", Reuters reported.

Unrest spread, with rioters targeting mosques and smashing windows of hotels housing asylum-seekers from Africa and the Middle East, chanting "get them out", in the first widespread outbreak of violence in Britain for 13 years.

Messages online said immigration centres and law firms aiding migrants would be targeted on Wednesday, prompting anti-fascist groups to say they would counter any demonstration.

Speaking after an emergency meeting with ministers and police chiefs on Tuesday, Starmer said police would be in place to cope with any further disorder.

"Our first duty is to ensure our communities are safe," he told broadcasters. "They will be safe. We are doing everything we can to ensure that where a police response is needed, it is in place, where support is needed for particular places, that is in place."

He said the fact that protests were being held in multiple locations made it difficult, but he had received the assurance he needed that police could cope with any disorder.

The government has increased prison capacity to cope with the large number of arrests made during the riots, which have prompted a growing number of countries to warn their citizens about the dangers of travelling in Britain.

Starmer said more than 400 people had been arrested, 100 had been charged, and he was expecting sentencing to start soon.

"Anybody involving themselves in this disorder is going to feel the full force of the law," he said.

Three people will be sentenced on Wednesday in Liverpool, northwest England, after pleading guilty to violent disorder, the Crown Prosecution Service said.

The justice department, which is due to release some prisoners early as it battles a jail overcrowding crisis, said nearly 600 prison places had been secured to accommodate those engaged in violence.

The unrest has prompted India, Australia, Nigeria and other countries to warn their citizens to stay vigilant.