A group of up to 20 people looked on as a gang punched and kicked a teenage asylum seeker at a bus stop in south London, neighbours have said.
Eight people are being questioned by police on suspicion of the attempted murder of the Kurdish Iranian victim following Friday's attack in Shrublands Avenue, Croydon.
Those involved in the attack have been labelled "scum" by the local MP.
London Mayor Sadiq Khan also condemned the attack.
Detectives made three further arrests on Sunday - two men aged 23 and 26 and a 17-year-old girl.
Five others who were arrested on Saturday - four men and one woman, all aged 20 or 24 - remain in custody.
A 20-year-old woman arrested on Saturday has been released with no further action, the Metropolitan Police said.
One resident of Shrublands Avenue said: "There was a massive group coming up from The Goat pub.
"You couldn't see who was hitting who. There was one person, the one who ended up in hospital, he was getting absolutely beaten up.
"There was a group of roughly 10 people kicking and punching him and the rest, another 10 or 20, were all just around watching."
"People that were there, witnesses, because they couldn't do anything against a group of 30 people, they had to wait until they moved off and that's when they managed to help him.
"That's eventually when the ambulance and police (arrived), and that was it."
The victim was waiting with two friends at the bus stop when he was set upon at about 23:40 BST.
It is believed the attackers asked him where he was from before they chased him down the road and repeatedly kicked him as he lay on the ground.
His friends suffered minor injuries in the attack, but the teenager is reported to have sustained a fractured skull and a blood clot on his brain.
He was taken to a south London hospital where he remains in a "serious but stable" condition.
Officers are in the process of informing next of kin.
At the scene
By Simon Jones, BBC News correspondent
As high-visibility police patrols in the area continue, residents in this diverse community have been trying to make sense of what happened.
The police tape around the bus stop has been taken down, transport is running normally again.
But as people sat waiting for buses, the attack was the only topic of conversation.
Many were keen to tell me what happened was not typical of the area, describing it as a place where people do get along.
A few though say it's left them feeling scared, particularly as more details have been released by the police about what happened.
The local MP, on his visit to talk to the numerous TV cameras, took time to speak to one of the community leaders, so they could share their sense of shock.
One resident has responded by spraying graffiti near the numerous flats on Shrublands Avenue that reads simply: "Refugees Welcome".
London Mayor Sadiq Khan said in a statement that there would always be a "zero-tolerance approach" to hate crimes of any type.
"Hate crime has no place in London, Britain or anywhere else," he said.
The Refugee Council said it was "disgusted" by the assault.
Director of advocacy, Dr Lisa Doyle, said: "We hope the perpetrators of this appalling attack are swiftly brought to justice and we wish the victim a full and speedy recovery."
Kana Varathan, who lives near the scene of the attack, said he heard the screams of the victim.
He said: "Normally, Friday night, they are always making noise, then it sounded really funny.
"Then, looking to the window, the group of people [were] beating one person."
Croydon Central MP Gavin Barwell used Twitter to appeal to anyone with information "about the scum" responsible for the attack to contact the police.
He told the BBC: "I described them on Twitter as scum. I think this is a cowardly and despicable attack.
"It appears that this young boy that sought sanctuary in this country has been set upon by a whole group of people.
"And I hope that we will find the people responsible and that they will face the full force of our criminal justice system."
Mr Barwell added: "The other thing that I would say is that what happened here is not representative of this community and that Croydon is one of the most diverse bits of London and people of different backgrounds get along very well."
Ch Supt Jeff Boothe said: "A number of bystanders and eyewitnesses tried to intervene and say to the attackers that 'enough is enough'."
He added: "Hate crime is something which we understand can be very, very divisive. Croydon is culturally diverse and we need to continue to celebrate that.
"We are appealing to all decent people from whatever background they come from to help us identify the individuals that are involved in this isolated attack."