Witnesses say an attacker laughed as he was arrested by police officers after a van “intentionally”’ swerved into a crowd of pedestrians near London’s Finsbury Park Mosque.
Muslim man Rabie Rafa, 42, said the man was jovial as he was arrested.
"He just laughed,” he said.
"They just killed people and he laughed.
"He had just ran people over up the sidewalk and killed innocent people.
"It was horrible to witness."
London police say one person has been detained after the incident, which caused "a number of casualties" just after midnight local time.
Eyewitness Zakriya James, 24, said he believed there were four attackers in what appears to have been a rented white delivery van. Three got away.
The local said the van "ploughed" into people as they exited the mosque.
"The van just came through as people were walking towards where it was coming from," he said.
"They didn’t realise what was going on at first. People just started running.
"As far as I know there were four white males and three got away.
"It probably is a retaliation. People were coming out of their prayer. That says something."
Reports suggest up to 10 people were hurt. The Sun reported that two are feared dead.
Up to 100 hundred people gathered outside the cordoned off zone blocking the mosque, with tensions high after the incident.
A group of men huddled together to condemn the "terror attack" outside the mosque, where people had been engaging in Ramadan.
"This is a terror attack!" a man shouted.
One person has been arrested.
There is a massive police response on the scene with riot vans, armed personnel and a helicopter.
A man, who asked not to be named, said his uncle had been hit by the van.
"I have not heard from him but my friend told me he’d been hurt," he told News Corp.
"The van just drove up and hit them out of nowhere.
"They didn’t see it coming.
"They knew there would be people there.
"They were white extremists."
Photos purporting to show the white van used in the attack show it was rented from a company in Wales.
The Muslim Council of Britain, said on Twitter: "We have been informed that a van has run over worshippers as they left #FinsburyPark Mosque. Our prayers are with the victims.
Harun Khan, the general secretary of the MCB, tweeted that the van had "intentionally" run over people leaving night prayers held during the holy month of Ramadan.
Khan said he was "shocked and outraged to hear a van has intentionally run over worshippers leaving Ramadan prayers".
Mohammed Shafiq of the Ramadhan Foundation, a Muslim organisation which speaks out against extremism, said: "I utterly condemn the senseless and evil van attack against Muslim worshippers outside the Finsbury Park Mosque in London. "According to eyewitnesses this was a deliberate attack against innocent Muslims going about their life.
"We should make clear that if this attack is confirmed as a deliberate terrorist attack then this should be classed as an act of terrorism.
"The British Muslim community requires all decent people to stand with us against this evil violence.
"Rampant Islamaphobia has been on the rise for a number of years and those on the far right have perpetuated hatred against Muslims.
"They should be called out for their hatred."
Eyewitnesses have told LBC radio that the van approached "slowly and intentionally accelerated into crowded cafe outside a mosque".
Another witness told the station: "I saw police giving CPR, getting the heart going again and another guy on the floor."
Local resident Mohammed Mudhar, 28, said his family were inside the mosque at the time.
"I ran here to check they were OK," he said.
"Luckily they weren’t hit.
"They did this at a time they had thought through perfectly.
"It was right as they had finished prayer and were coming out to break their fast and eat.
"They hit about 10 people. The timing was spot on.
"It makes me very scared. We’re just normal people."
A witness told LBC radio people had been drinking coffee at a nearby cafe after leaving the mosque after Tarawih prayers performed for the Islamic holy month of Ramadan.
The holiest month of the Islamic calendar, Ramadan will finish this coming Saturday, June 24.
Another caller told LBC her sister was at the scene when the incident happened.
The woman said her sister described it as "something from a horror movie, everyone running everywhere".
She didn’t realise what was going until she saw people on the ground.
One witness told the BBC at least three people were seriously injured. "From the window, I started hearing a lot of yelling and screeching, a lot of chaos outside. ... Everybody was shouting: ’A van’s hit people, a van’s hit people’," a woman who lives opposite the scene told the BBC. "There was this white van stopped outside Finsbury Park mosque that seemed to have hit people who were coming out after prayers had finished. I didn’t see the attacker himself, although he seems to have been arrested, but I did see the van."
Eyewitnesses reported seeing bystanders wrestle a suspect to the ground and pin him down until officers arrived.
Mohammed Kozbar, chairman of Finsbury Park mosque, told The Sun: "Whoever did this, he did it to hurt people and it’s a terrorist attack.
"We call it a terrorist attack as we called it in Manchester, Westminster and London Bridge. The van drove over people who were leaving late prayers. They were gathering in Seven Sisters Road.
"I heard there are fatalities but I don’t want to jump to conclusions. My concern now is the casualties who have been affected."