Witnesses said they saw people at the scene grab the man, pulling him from the van and started hitting him.
Commissioner Dick said: "This was quite clearly an attack on Muslims who looked like they were probably Muslims and they were coming from a prayer meeting". She added that the attack had "once again targeted the ordinary and the innocent going about their daily lives - this time, British Muslims as they left a mosque, having broken their fast and prayed together at this sacred time of year". According to Metropolitan police, the injured were taken to three different hospitals.
Police said they were not looking for any more suspects after the arrest of the van's driver.
Those leaving the mosque captured the alleged assailant and pinned him to the ground.
London Mayor Sadiq Khan said "terrorism is terrorism".
Others linked the attack to an increase in anti-Muslim hate crimes, particularly since the London Bridge rampage on June 3 that left eight people dead, which was claimed by the Islamic State group. Counter-terror police were probing the incident.
One man was trapped under the van and people tried to lift up the vehicle to free him.
Mohammed Kozbar, chairman of the Finsbury Park Mosque, called for Prime Minister Theresa May's government to take action. The elderly man died at the scene, but it wasn't immediately clear whether he received additional injuries from the van or if his medical treatment was cut short by the incident.
"Our heart goes out to the people of London who have been through so much in the last few months and are still waiting for news on many friends and family in the Grenfell Tower tragedy".
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. "All my thoughts are with those who have been injured, their loved ones, and the emergency services on the scene".