After the protests on Sunday became more violent and a mob tried to set fire to a hotel that was sheltering asylum seekers, Keir Starmer has sent out a strong warning to "far-right thugs," stating they will regret engaging in violent disturbance.

The prime minister declared that "all right-minded people should condemn" the disruption and that "violent rioters who attack people based only on the color of their skin will be swiftly prosecuted.

"There is no doubt about it: everyone involved in this violence will be held fully accountable by the law," he declared. "Arrests will be made by the police. People are going to be kept under remand. There will be charges. Convictions will then come. You will regret engaging in this disease, I promise.

Speaking from Downing Street, he pledged to those feeling frightened about being targeted because of their race or religion that “this violent mob do not represent our country and we will bring them to justice”.

Starmer will lead a Cobra meeting on Monday, as Labour, Conservative and Lib Dem politicians condemned the scenes of violent gangs attacking the Holiday Inn Express in Rotherham in South Yorkshire where asylum seekers were housed. A group of rioters in Middlesbrough in the north-east also walked through a residential area smashing the windows of houses and cars.

At least 247 arrests have been made over the weekend across England and Northern Ireland, including in London, Hartlepool, Bristol, Belfast, Southport, Hull, Stoke-on-Trent and Liverpool, and some charges have been brought, with police warning of more to come once footage has been scoured.

The violence is the worst mass disorder since the 2011 riots across England, with police dealing with 47 gatherings from the far right and counter-protesters on Saturday, and nine on Sunday.

Chief constable BJ Harrington, the national lead for public order, told the Guardian a range of criminal offences were being investigated, from those committing violence on the streets, to those alleged to be whipping it up: “We are looking at a whole range of stuff. If we find there are people conspiring to commit violence, we will bring charges.

“Social media is playing a large part, we are looking at that,” he said, adding it was being used in the “incitement and encouragement”.

He said: “People in foreign countries are putting out disinformation.”

Asked if that was a reference to the former EDL leader, Stephen Yaxley-Lennon who uses the name Tommy Robinson, the police chief said: “Tommy Robinson is one among many.”

Harrington said, with 4,000 riot-trained officers pulled out of regular policing, “it is an obvious concern” that regular policing will suffer.

Muslim leaders are warning their communities no longer feel safe, and the Home Office on Sunday offered new emergency security measures aimed at protecting mosques. It will involve fast-tracking the provision of security personnel available under a current scheme.

The surge in extreme right-wing activity in the past week has led to a fivefold increase in threats to Muslims, such as rape and death, and a threefold increase in hate crime incidents, according to Tell Mama, a national monitoring group. It said Muslims in Britain have been left “terrorised” by the increase in extreme right-wing activity since Monday, which is directly linked to a large increase in anti-Islamic hate crimes.

In a broadcast to the nation from No10, Starmer said those taking part, whether in person or “whipping it up online and then running away themselves”, would regret their actions.

Asked whether he thought everyone taking part in the riots were “far-right thugs”, he said: “If you target people because of the colour of their skin or their face then that is far right and I’m prepared to say so. But it doesn’t matter what apparent motivation there is. This is violence, not protest. It doesn’t matter what the motivation is.”