Politicians from across the spectrum have called for calm after a knife attack in Belfast prompted widespread shock and condemnation.

There are fears that there could be widespread disorder after figures on social media including Elon Musk called for people to fill the streets in protest against immigration. The alleged perpetrator of the attack, which was filmed and shared widely online, was revealed today as an asylum seeker from Sudan.

The attack happened at about 10.30pm on Monday outside a block of flats in north Belfast. Graphic video shared on social media showed a man straddling another man on the ground and striking at his head and neck.

The clip showed people intervening to stop the assault, with one man, later named locally as Maitiu Mag Tighearnan, using a hurling stick against the attacker multiple times.

Police said the arrested man was believed to be Sudanese and in his 30s. They had initially said he was thought to be from Somalia. The victim, who has not been named, is in his 40s.

As of Tuesday evening, the suspect was in custody and the victim was in a serious condition in hospital. Police said the victim had serious injuries to his eyes, and serious slash wounds to his back and face.

Figures from across the world have taken to social media to call for protests against immigration. Some posts from accounts in Northern Ireland announce that roads are “closed” for a protest and warn that all businesses in the area should shut at 5.30pm to prepare for disorder.

Stephen Yaxley-Lennon, a far-right agitator who refers to himself as Tommy Robinson shared the video of the attack on Monday night and posted a call for protests in central London and elsewhere in the UK.

Sudanese business owners on Sandy Row, a loyalist area of central Belfast, closed their stores with steel shutters by 4pm and said they planned to stay at home that night.

The Belfast Islamic Centre cancelled evening prayers and said police advised them that the next 24 hours would be crucial. “We are telling our congregation to go home, don’t go out, look after your children, don’t share rumours and do listen to the authorities,” said Ameer Ibrahim, a project manager who spoke in a personal capacity.

The first minister, Michelle O’Neill, told the public not to be persuaded by social media accounts to start causing disorder. She said: “For all of those people out there who are stoking up tensions in that social media space who are happy to raise tensions, they do not represent us. We are good people and I don’t want to see anybody living in fear.”

The deputy first minister, Emma Little-Pengelly, added she was issuing a “plea for calm”, acknowledging that people would be feeling a “bag of emotions” but added: “don’t allow those people who don’t care about people here to incite hatred and incite fear”.

Rightwing commentators from England and the US, including the MP Rupert Lowe and billionaire and owner of X Elon Musk have been posting about the attack. Musk shared a list of potential protest areas in the UK and wrote “Only by protesting REPEATEDLY and LOUDLY will there be any change!!”

Jon Boutcher, the chief constable of PSNI, told a press conference on Tuesday: “We are aware of course of protest activity being planned across Northern Ireland tonight. We understand that people will be feeling enraged with emotions … but please, please let the PSNI do their job unfettered and undistracted from wider concerns there may be about disorder.” He added: “The challenge we face with today’s online toxic nature is that people are incited by people who are faceless and know nothing about this brilliant vibrant place. Do not be fooled or duped by people online.”

There was much discussion and speculation over the alleged perpetrator’s immigration status. He is understood to have a five-year visa after travelling from Dublin to Belfast via bus and claiming asylum. In a social media post, Nigel Farage, the Reform UK leader, said authorities must immediately disclose the suspect’s identity and immigration status.

Little-Pengelly said: “The UK must be able to deport these people much more swiftly than they have thus far. Importantly, people must know their legitimate concerns are listened to. Communities must feel that the protections that are in place are working to keep them safe”

The assistant chief constable, Ryan Henderson, told reporters the suspect was in the country legally.

It is understood that the PSNI has held emergency meetings to draw up plans for how to deal with any unrest, after far-right figures online called for people to take to the streets wearing masks.

Henderson said there would be an “increased police presence” across Northern Ireland in case of unrest. “People will feel a range of emotions from fear to anger,” he said.

The Northern Ireland secretary, Hilary Benn, said that protests were “not going to help anyone” because they would “stretch police resources”.

Reform UK’s home affairs spokesperson, Zia Yusuf, said: “The horror of what you have seen in Belfast is a direct result of treacherous Tory and Labour immigration policy.

“Reform has already announced a total ban on visas for anyone from Sudan. Enough is enough.”

Keir Starmer called the attack horrific and sickening. “I have absolutely no tolerance for abhorrent scenes of violence like this on our streets. My thoughts are first and foremost with the victim, and I thank the first responders, including members of the public, who intervened.”