Police in Northern Ireland appealed for calm after arresting a Sudanese man on Tuesday over a knife attack that left one person in a serious condition and prompted calls online for a protest.
British Prime Minister Keir Starmer described the incident as "sickening" after video of the attack, which took place in north Belfast late on Monday evening, was shared widely on social media.
The incident, which is currently not being treated as terrorism, comes at a time of heightened tensions in Britain following the murder of a student who was handcuffed by police as he lay dying from stab wounds after his killer, a Sikh man, falsely alleged a racist attack.
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It also follows repeated protests about immigration, with populist parties saying Britain's asylum policy had allowed dangerous men into the country. Northern Ireland saw anti-immigrant rioting last year amid anger over an alleged sexual assault.
"I understand that last night's attempted murder will leave people feeling a range of emotions, from fear to anger," Northern Ireland's Assistant Chief Constable Ryan Henderson, who has declared it a "critical incident," told a press conference.
"I appeal for calm and the safety of all of our communities in response to this."
He said the victim, a man in his 40s, suffered significant injuries to his eyes and slash wounds to his face and back during the "brutal" attack, which involved a kitchen knife.
Footage showed a number of members of public trying to fight off the attacker before police arrived on the scene.
The suspect, a 30-year-old Sudanese national, had been detained on suspicion of attempted murder. Henderson said he understood the man lived locally, having been granted leave to remain in the province after traveling there from Dublin.
Northern Ireland's main political party leaders jointly condemned the attack in a statement, calling it "horrific," also urging calm while backing the police investigation.
The deputy first minister, Emma Little-Pengelly, had described it as a "savage and barbaric attack."
A widely shared post on Facebook and WhatsApp called for protests on Tuesday evening, and Henderson said that, while people had a right to have their voices heard, that could be done peacefully.
"Nobody, nobody in Northern Ireland wants to see a repeat of the disorder that we have seen in recent years," he said.