London (AFP) - UK Prime Minister Keir Starmer warned Friday that authorities must “stay on high alert” for more far-right riots, as courts issued the first jail sentences for online incitement during the recent disorder.

Starmer’s comments followed consecutive nights of relative quiet across England, after a week of near daily unrest in more than a dozen places after a knife attack that killed three children.

Disturbances have continued unabated in Northern Ireland, however, where police have blamed pro-UK loyalist paramilitaries for fuelling nightly violence in Belfast.

More than 1,000 anti-racism protesters massed in the Northern Irish capital early evening Friday amid a large police presence.

Several dozen anti-immigration demonstrators soon also showed up, with officials braced for further violence in the city.

In London, Starmer told reporters during a visit to the headquarters of the Metropolitan Police that “swift justice” was helping deter more disorder in English towns and cities.

“I’m absolutely convinced that having the police officers in place these last few days, and the swift justice that has been dispensed in our courts have had a real impact,” he said.

“But we have to stay on high alert going into this weekend because we absolutely have to make sure that our communities are safe and secure.”

- ‘Reminder’ -

Hours later, a judge in Leeds, northern England, jailed a 28-year-old man for 20 months after he admitted publishing Facebook posts that met the criminal threshold for stirring up racial hatred.

In the first case of its kind linked to the disturbances, the judge sentenced Jordan Parlour for the posts last week encouraging people to attack a hotel in the city housing asylum seekers and refugees.

The hotel manager had to put the building into lockdown Saturday due to the disorder in the city, and at least one window was broken after stones were thrown at it over the weekend.

In Northampton, central England, a judge jailed 26-year-old Tyler Kay for 38 months after he called on social media for hotels housing asylum seekers to be set alight.

Speaking before the sentences – but after both had been convicted – Starmer said they were “a reminder to everyone that whether you’re directly involved or whether you’re remotely involved, you’re culpable”.

Social media executives as well as users should be “mindful of the first priority, which is to ensure that our communities are safe and secure”, he added, hinting at stricter enforcement in future.

“We’re going to have to look more broadly at social media after this disorder but the focus at the moment has to be on dealing with the disorder,” Starmer said.

- Nearly 600 arrests -

The UK leader held another crisis meeting of top ministers and police chiefs late Thursday.

Police in England said Friday that forces nationwide had now arrested nearly 600 people suspected of taking part in the unrest that began on July 30, and around 150 charges had been filed.

The disturbances, first sparked by a July 29 knife attack in which three children were killed, have seen mosques and migrant-related facilities attacked alongside police and other targets.

Officials say false information spread on social media about the suspected perpetrator fuelled the disorder.

Courts across England have started sentencing convicted participants in the disorder, with some facing years in jail. Already on Thursday round a dozen people were jailed.

In Northern Ireland, a number of Belfast businesses and libraries announced plans to close early on Friday after more disorder overnight and further planned protests.

A “significant visible policing operation” will be in place ahead of anti-immigration protests and counter-protests planned in Belfast’s city centre.

Police there said 23 people have been arrested so far following the disturbances, and 15 charged.

Officers have been granted additional powers to stop and search suspected troublemakers and ask them to remove face coverings, while additional manpower is being sent from the UK mainland, according to reports.