Swift justice has been a deterrent to more violent disorder after riots in the past week in parts of the UK, Sir Keir Starmer has said.

The prime minister told an emergency meeting of Cobra on Thursday however that police needed to remain on “high alert”.

There does not appear to have been new anti-immigration protests overnight. Unrest erupted last week in England and Northern Ireland after three girls were killed in Southport and misinformation spread online that the suspect was an asylum seeker.

More than 500 people have so far been arrested, with more than a dozen convicted of offences and sentenced. A quarter of those charged are under the age of 21.

Sir Keir was England and Wales’s top prosecutor during the last significant riots in 2011 and there would have been lessons he would have learnt there that he is likely to be trying to implement including getting public order officers in the right places.

Riots broke out across England last week, from Plymouth on the south coast to Sunderland in the North East and also in Belfast, in Northern Ireland.

Crowds were seen attacking mosques; accommodation housing asylum seekers, cars and buildings including a library were set on fire; and shops were looted.

Many police officers were injured, with more than 50 suffering broken bones, concussion and head wounds during a single incident at the Rotherham hotel riot.

So far, 149 charges have been brought with police expecting the number to “rise significantly” as suspects are fast-tracked to appear in court.

Tough sentences will continue with maximum publicity to try to deter future violence, the BBC understands.

Almost 6,000 extra public order officers mobilised earlier in the week remain in place.

The Mayor of London, Sadiq Khan, urged the government to revisit social media rules, saying the recent disorder showed regulations due to come into force were “not fit for purpose”.

The Online Safety Act will, for the first time, make firms legally responsible for keeping users safe when they use their services.

It will require platforms to put in place clear and proportionate safety measures to prevent illegal and other harmful content from appearing and spreading on their sites.

Platforms will be required to take “robust action” against illegal content and activity, including around offences such as inciting violence.

The biggest platforms could face billions of pounds in fines if they do not comply.

After the stabbings in Southport, an incorrect name and a false story around the background of the suspected perpetrator spread online.

Mr Khan said this misinformation spread on social media showed reform of the regulation was needed.

“The way the algorithms work, the way that misinformation can spread very quickly and disinformation … that’s a cause to be concerned, we’ve seen a direct consequence of this,” he told the Guardian.

In recent days Elon Musk, the owner of X, has been heavily criticised for posts about the disorder in the wake of the Southport stabbings.

The billionaire was called “deeply irresponsible” by Justice Secretary Heidi Alexander for posting that “civil war is inevitable” in the UK.

He also reposted an image of a fake news headline about the UK’s response to riots.

Gavin Stephens, chairman of the National Police Chiefs’ Council, has warned that those “intent on violence and destruction have not gone away”.

“We’ve seen on the footage that we get back from the scene, young people, children, are on the fringes of it, and in that mob mentality can get drawn in,” he said on Thursday.