A man believed to be Somali has been arrested on suspicion of attempted murder after a "brutal" knife attack in Belfast, police have said.
The man, in his 30s, remains in custody after the incident in north Belfast at about 22:30 BST on Monday.
Video circulating online shows a number of people, including one wielding a hurling stick, confronting the apparent attacker until the police arrived.
A man injured in the attack, aged in his 40s, is in hospital where his condition is described as serious.
Prime Minister Sir Keir Starmer called the incident "sickening", adding that he will have "absolutely no tolerance for abhorrent scenes of violence like this on our streets".
"The horrific attack in Belfast last night is sickening," the prime minister said.
"My thoughts are first and foremost with the victim, and I thank the first responders, including members of the public who intervened."
Emergency services were called to Kinnaird Street in north Belfast on Monday night.
The victim sustained significant injuries to his face, neck and back.
Police Service of Northern Ireland (PSNI) Assistant Chief Constable Ryan Henderson said "a critical incident" has been declared and police have begun an investigation to establish a motive.
"This brutal attack will have sent shockwaves through the community causing real concern.
"I want to reassure the local community that we are treating this attack with the utmost seriousness.
"Our investigation is continuing at pace."
Henderson said police were on the scene within minutes and acknowledged the members of the public "who strived to save the man from further attack".
He said they demonstrated "bravery and community spirit".
He asked that people do not share footage of the attack.
The Northern Ireland Ambulance Service (NIAS) said one emergency ambulance and an advanced paramedic were sent to the scene and the victim was then taken to the Royal Victoria Hospital.
A resident told BBC News NI how she was in her home when her partner heard screaming outside.
She said others, including children, had begun to gather in the street and described how some people tried to intervene.
The woman said she had "never seen anything like it".
"The police came and I kept my kids in the house; I was just absolutely petrified.
"I feel frightened."
First Minister Michelle O'Neill described the attack as "harrowing".
"My thoughts are with the injured man and all those who responded at the scene.
"I also want to recognise the bravery of members of the local community who put their own safety at risk in an effort to stop this vicious attack."
O'Neill said she has spoken with the chief constable of the Police Service of Northern Ireland (PSNI) and urged people to give police "the space it needs to carry out a full investigation so that justice can be done".
Northern Ireland's Justice Minister, Naomi Long, condemned the attack and said "there is no place for this kind of horrific violence in our community".
She said this was a time to "remain calm and lawful and allow the police to do their job".
"I don't think its helpful, for people to seize on this as yet another weapon, in the war that they wage on issues around immigration, I do not think it is healthy and I do not think it is fair."
Leader of the Democratic Unionist Party (DUP), Gavin Robinson, said anyone who has seen video footage of the attack "will be unable to unsee it" and it was "barbaric".
He said "this issue has a different sense to it and there are concerns about the consequences of this incident", and said it is "essential police speak clearly on the incident".
Deputy First Minister Emma Little-Pengelly said people across Northern Ireland were "sickened and appalled" by what had happened.
She said she has spoken to the PSNI Chief Constable and the justice minister to reiterate the importance of "clear communication" with the public.
"There can be no excuses and no tolerance for such violence".
"I know emotions are running high - there is anger and concern but I do urge everyone to remain calm to allow the police to carry out their investigation and allow people to establish the full facts."
Little-Pengelly said she had also been in contact with the prime minister to ensure "full and urgent cooperation" with the Home Office to see what information they hold in relation to the man who has been arrested in connection with the attack.
Leader of the Conservative Party, Kemi Badenoch, said she was "absolutely horrified" by the attack.
"I think a lot of people will start wondering yet again, is this someone who should not have been in our country? Have there been failings around our borders?
"I would urge the police to bring the facts out as quickly as possible so that we can get some clarity," she said.
Leader of the Ulster Unionist Party (UUP) Jon Burrows said the incident was "beyond any attack I've seen in the likes of Northern Ireland before".
He expressed concerns around the narratives that could play out in the coming days, and said people may attempt to exploit the incident.
"It is important that the police come forward as early as possible with clear, factual and objective information, and that we do not allow any vacuum to be filled with disinformation."
Jim Allister, leader of the Traditional Unionist Voice (TUV), said the "only bright spot in this dark orgy of violence" was the group of locals who stopped the attack.
"Cool heads, of course, are needed, but the police and the government must be shown to be effective."
Assembly member for Belfast North, Nuala McAllister, has appealed for people to allow "police to do their job".
"We have had attacks before and people have died due to stabbing but they have happened behind closed doors, the fact that this is caught on camera, it shocks people more, but we need to allow the police to do their job."
Paul McCusker, an independent councillor in the area, said "one lady said she had to go to hospital herself because of the stress of witnessing such a brutal attack".
He praised the "brave residents" who intervened to stop the attack.
A statement from the North West Migrants Forum said they are "absolutely horrified" by the news and recognise that people will be "traumatised" by what they witnessed.
It added: "Obviously attacks like this do not make our job any easier and we understand completely that many people will feel frightened, angry and in want of answers.
"Our members are also frightened. Good, decent, honest men and women fretting this morning about what last night's attack will mean for them."
