Police in Northern Ireland have arrested a man on suspicion of attempted murder and declared a critical incident after a stabbing in Belfast.

The suspect was in custody and the victim was in a serious condition in hospital after the attack on Monday night that prompted widespread shock and condemnation.

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.”

The attack happened at about 10.30pm on Monday outside an apartment complex 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 using a hurling stick against the attacker.

It is understood there is no indication at this stage that the attack had a terrorist motive.

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

The assistant chief constable Ryan Henderson said police had declared a critical incident and were investigating the motive. “This brutal attack will have sent shock waves through the community, causing real concern. I want to reassure the local community that we are treating this attack with the utmost seriousness.”

Henderson said officers were at the scene within minutes. “We wish to acknowledge the members of the public who strived to save the man from further attack. Their willingness to step forward to help another person shows incredible bravery and community spirit.”

Henderson urged people to not share or repost images or footage of the assault. “Doing so risks causing further trauma to the injured man’s loved ones and may impact the ongoing investigation.”

Naomi Long, the Stormont justice minister, echoed the appeal, saying the images were “deeply disturbing and traumatising to view”.

John Finucane, the Sinn Féin MP for north Belfast, visited the scene on Tuesday morning and called the attack “appalling”.

Gavin Robinson, the Democratic Unionist party leader, asked police to clarify the circumstances of the incident to combat misinformation. “This was medieval,” he told the BBC. “This was a systematic mutilation of a human being on the streets of Belfast.”

In a social media post, Nigel Farage, the Reform UK leader, said authorities must immediately disclose the suspect’s identity and immigration status.

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