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.
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 on Tuesday.
"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, with a kitchen knife found at the scene.
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, has been detained on suspicion of attempted murder.
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.
"Last night's attack in North Belfast was horrific and shocking. Many people are feeling angry, and many people are feeling deeply distressed," First Minister Michelle O'Neill told reporters.
A widely shared post on Facebook and WhatsApp has called for protests on Tuesday evening, and in response police have promised an increased presence across the region.
"Nobody, nobody in Northern Ireland wants to see a repeat of the disorder that we have seen in recent years," Henderson said.
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