British Prime Minister Keir Starmer described the attack, which took place in north Belfast late on Monday evening, as "sickening". Video of it was shared widely on social media.
The knife attack, 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.
It also follows repeated protests about immigration, with populist parties saying Britain's asylum policy had allowed dangerous men into the country. There was anti-immigrant rioting in Northern Ireland last year amid anger over an alleged sexual assault.
Masked youths gathered at points across the city, with police responding by deploying armoured vehicles. Protesters set fire to a number of vehicles, including a bus in east Belfast.
The BBC reported that a crowd of 100 men kicked in doors and broke windows of homes on a street in east Belfast. Sky News showed footage of a house on fire.
A few dozen protesters blocked Parliament Square in London.
"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."
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, with a kitchen knife found at the scene, Henderson said.
Footage showed a number of members of the public trying to fight off the attacker before police arrived, and they were credited by senior officers with saving the man's life.
The suspect, a 30-year-old Sudanese national, was charged on Tuesday evening with attempted murder, possession of an article with blade or point in a public place and threats to kill.
He is due to appear at Belfast Magistrates' Court on Wednesday.
Police said it was understood he lived locally, having been granted leave to remain in the UK in September 2023 after claiming asylum. He had travelled to Belfast in February that year by bus from Dublin, having flown there from Paris on an unknown date.
"There is no trace of this suspect on any of our national security databases, and he was not known to the Police Service of Northern Ireland," Chief Constable Jon Boutcher said.
Northern Ireland's main political party leaders jointly condemned the attack, calling it "horrific", and also called for calm, saying disturbances would only damage their communities.
