One witness described being surrounded by bodies in the wake of the attack outside the Muslim Welfare House, yards from the mosque.
The van's 48-year-old driver was detained by members of the public and then arrested by police on suspicion of attempted murder.
A van ran into a crowd of pedestrians outside a London mosque early on Monday, and there are "a number of casualties being worked on at the scene", according to police.
- Jeremy Corbyn, the leader of Britain's opposition Labour Party tweeted: "I'm totally shocked at the incident at Finsbury Park tonight".
Police say several people have been injured, with many receiving treatment at the scene. A police photographer records the scene at Finsbury Park in north London, where a vehicle struck pedestrians in north London Monday, June 19, 2017.
A van slammed into Muslim worshippers near a mosque in north London early this morning, killing at least one and injuring 10, British police and Islamic officials said.
"Whoever did this, he did it to hurt people and it's a terrorist attack".
"Hatred and evil of this kind will never succeed", she said in a televised statement. Police have not released his identity.
Someone in the crowd yelled to others not to harm the man while he was taken into custody.
"This is being treated as a terrorist attack", said Metropolitan Police Deputy Assistant Commissioner Neil Basu told reporters.
Mayor of London Sadiq Khan said extra police had been deployed to reassure communities, especially those observing Ramadan, describing the attack as "an assault on all our shared values of tolerance, freedom and respect".
The chairman of the Finsbury Park Mosque says the van crash that hit worshippers was a "cowardly attack" and urged Muslims going to mosques to be vigilant.
London police closed the area to normal traffic. But imam Mohammed Mahmoud stepped up and told the angry crowd to stop and to restrain the suspect until police arrived instead.
"If you were interested in targeting a group of Muslims at a mosque that was notorious, this would be the mosque you would do it at".
A witness, who wanted to be identified as Abdulrahman, said: "I managed to get the driver of the van". "This person was conscious".
"He was screaming "I'm gonna kill all Muslims, all Muslims".
One victim of the attack is believed to have died. But after that I heard people just screaming "terrorist attack, terrorist attack".
"I saw people taking a man from underneath the van, he was black, bleeding, he was not dead, he was alive".
Saadiq Mizou, a 35-year-old chef originally from Belgium, said he'd been outside the scene since 2 a.m.
This is post will be updated as more information becomes available. For him, the attack had made him reconsider whether he could go to the mosques in Finsbury Park again. It has not been associated with radical views for more than a decade.
Opened in 1994, Finsbury Park Mosque is an unassuming five-story redbrick building in residential north London, close to Arsenal Football Club's Emirates Stadium.
Harun Khan, Secretary General of the Muslim Council of Britain, said many will feel "terrorised, no doubt be angry and saddened" after the attack.
Earlier this month on London Bridge, attackers used a vehicle and then knives to kill eight people and wound many others on the bridge and in the nearby Borough Market area.
Mr Rahman told the BBC he struck the man and helped subdue him.