According to one man at the scene, the suspect - a 48-year-old who was detained by onlookers before being arrested by police - screamed "I'm going to kill all Muslims" before he was tackled to the floor.
Abdulrahman Saleh Alamoudi said he was among a group of people helping an elderly worshipper who had fallen down, perhaps because of the heat, when the van swerved towards them.
He told BuzzFeed News: "This big van just came and went all over us.
"I think at least eight or 10 people got injured. Luckily I managed to escape. And then the guy came out of his van and I got him.
"He was screaming, he was saying, 'I'm going to kill all Muslims, I'm going to kill all Muslims'. He was throwing punches.
"Then we managed to get him on the floor. Then he was saying, 'Kill me, kill me'. I said, 'We are not going to kill you. Why did you do that?' He wouldn't say anything."
Another witness, Abdikadar Warfa, said he helped to detain the suspect while his friends helped victims.
"I saw a man underneath the van. He was bleeding. My friend said he had to lift the van, I was busy with a man who tried to escape. My friend said he said some words, but I didn't hear it," he said.
"They (people who were hit) were mostly young. They are very bad. I tried to stop him (the suspect), some people were hitting him but I said stop him and keep him until the police came.
"He was trying to run away but people overpowered him. He was fighting to run away."
Mr Salah Alamoudi said he had also helped to hold the suspect on the ground for up to half an hour before police arrived.
"The guy, I had to keep him at least half an hour. He was a strong guy. A big man," he said.
"It was heartbreaking. It wasn't an accident."
One resident told the Press Association he jumped out of the way as the van struck pedestrians.
The man, who did not want to be named, said: "The gentleman went straight down this road, people were just conversing, talking, just doing what we're doing. And he just came into all of us.
"There was a lot of people. We got told to move straight away. I was shocked, shocked, shocked. There were bodies around me.
"Thank God I just moved to the side, I just jumped. Everyone is hurt. Everyone is actually hurt."
Another man, who gave his name as Mohin, said his cousin was caught up in the incident.
The 32-year-old told the Press Association: "There were a lot of people in the street, crossing the street, going to mosque for prayers, for forgiveness, just to pray and go home and eat.
"They are doing their usual daily routine - they did not expect a van coming out of nowhere hitting them.
"It was around 11.30pm, 12-ish. I've been living here all my life. As a community, everyone's a family here.
"I initially thought it was my cousin."
Andrew, a courier who did not wish to give his surname, said he saw three people on the floor and at least one of them appeared to be in Muslim dress.
The 45-year-old, of St Albans, Hertfordshire, said he was driving back from a night shift when he saw the aftermath of the collision.
He said: "When I drove past slowly I could count three people on the floor and police were performing CPR on one of them.
"The guy having CPR performed on him was in a gown, ethnic clothing."
Ratip Alsulaimen told the Press Association he was nearby when alerted by a friend.
He said: "I was sitting in the coffee shop just a couple of minutes away. One of our friends was reporting what was going on in the front of the mosque.
"He said the car was smashing into the people getting out of the mosque, so we just ran away, all of us, and came to see what's happening.
"When I came I saw ambulance people taking people to the ambulance. I think between eight to 10 people were taken away.
"There was an old man - about 60 years old - I don't know if he's alive or dead. There were other people, mid-30 to 40. There are men and women.
"We are shocked when we heard the news because we were just having a good time. We were praying for peace and for Grenfell Tower.
"When we heard this we were actually shocked."
Hamza Nimane, who was in the mosque at the time of the attack, told BBC Radio 4's Today programme: "It wasn't the police that arrested him - it was the people that came out of the mosque.
"They're the ones that grabbed him and managed to hold him down."
He added: "There were at least 300 people in the mosque praying, and everyone was panicking, everyone was screaming.
"I was looking around me, I saw a few people lying on the floor with blood on their head, and then obviously the ambulance service came and started treated them.
"There was people lying everywhere, basically; I saw at least seven people lying on the floor.
"Some of them even looked dead to me, to be honest, because people were trying to cover them with sheets and stuff."