Mobile phone footage taken by one of those who crowded round the police van seems to show the arrested 48-year-old man blowing a kiss at the person recording the video.
Other witnesses claimed the suspect was laughing after the attack.
A man who wanted to be identified as Abdulrahman, which is not his real name, said he helped detain the suspect and added: “When he went into the [police] van he made gestures, he was laughing.”
The 48-year-old was arrested after a hired van ploughed into pedestrians shortly after midnight, as worshippers left the nearby Finsbury Park mosque and Muslim Welfare House following Tarawih prayers performed for the Islamic holy month of Ramadan.
— samira (@hooyomataalo) June 19, 2017
One man is thought to have died, and 10 others were injured, with eight being taken to hospital.
The event is being treated as a terrorist incident and is being investigated by the Metropolitan Police’s Counter Terrorism Command.
Theresa May is due to chair a meeting of the government’s Cobra emergency committee in response to what happened.
Witness Abdul Rashid, 18, told The Independent that in the immediate aftermath of the incident, the suspect appeared calm and detached.
"He wasn't distressed, not in the slightest,” said Mr Rashid. "Everyone was shouting but he was completely desensitised. He looked blank.”
“The imam protected him to make sure he was conscious so police could apprehend him."
— Hamza Sharif (@HamzaJourno) June 19, 2017
Muslim Council of Britain secretary general Harun Khan condemned the attack and also urged calm.
He said: "During the night, ordinary British citizens were set upon while they were going about their lives, completing their night worship.
"My prayers are with the victims and their families. It appears from eyewitness accounts that the perpetrator was motivated by Islamophobia.
"Over the past weeks and months, Muslims have endured many incidents of Islamophobia, and this is the most violent manifestation to date.
"Given we are approaching the end of the month of Ramadan and the celebration of Eid with many Muslims going to local mosques, we expect the authorities to increase security outside mosques as a matter of urgency."
He added: "Many will feel terrorised, no doubt be angry and saddened by what has taken place tonight.
"We urge calm as the investigation establishes the full facts, and in these last days of Ramadan, pray for those affected and for justice."
Finsbury Park mosque used to attract headlines as the stamping ground of “hook-handed preacher of hate” Abu Hamza.
But it was raided and shut down and later reclaimed by the local Muslim community, who have transformed it into a place which actively promotes better community relations across faiths.
In 2015 it was the target of an attempted arson when a white man with a jerrycan full of petrol tried to light it and throw it over the railings in front of the site before fleeing on a moped.