The UN rights chief on Wednesday said that anti-immigrant violence rocking Belfast and recent clashes with police in Southampton were "shocking", criticising "incitement" on social media and elsewhere.

Authorities in Northern Ireland have accused far-right activists -- including US tech billionaire Elon Musk -- of stoking anti-immigrant violence and divisions as rioters in Belfast torched vehicles and buildings late Tuesday, forcing families to flee their homes.

The disorder, sparked by a brutal knife attack allegedly committed by a Sudanese national, comes with tensions already high in the United Kingdom following skirmishes in southern England last week over the police handling of the murder of a white student by a British Sikh man.

"Dehumanisation of whole groups within a society is totally unacceptable and is frankly despicable," Volker Turk told reporters in Geneva.

"The violence that comes out as a result in... both Northern Ireland and in Southampton were really shocking," he said.

"Providers" on social media platforms, he insisted, must "take the responsibility seriously that dehumanisation, hate speech, violence and incitement to violence is unacceptable".

"We cannot tolerate this in today's world... The polarisation that we see is shocking."

Footage of the Belfast attack went viral on social media, with appeals for calm from police and lawmakers going unheeded in some areas of Northern Ireland.

Northern Irish political leaders and the police had urged people not to share the video, noting its "graphic nature would only serve to retraumatise those involved". 

But numerous social media accounts linked to so-called "patriots" shared the footage, urging people to "protest against mass immigration into their communities".

Immigration is a hot-button issue in Britain, and has helped fuel the rise of the hard-right Reform UK party, led by Nigel Farage.

The country frequently sees anti-immigration protests. Demonstrators clashed with police in Southampton on June 2 amid outrage over the murder of 18-year-old Henry Nowak in December.

He was handcuffed by police as he lay dying after his murderer Vickrum Digwa falsely accused Nowak of racially abusing him.