One person has been arrested following the incident.
Police are responding to reports of a van colliding with pedestrians outside a mosque in London, an incident officials said has resulted in "a number of casualties."
According to London's Metropolitan Police, one person has been arrested following the incident, which occurred around 12:15 a.m. local time Monday on Seven Sisters Road, near Finsbury Park station in the northern part of the city.
Few details of the attack have been revealed, and the identity of the suspect is still unknown. Authorities said it was too early to tell whether the incident was an act of terrorism.
London Mayor Sadiq Khan also used cautious language, describing the collision as "a major incident" in a post on Twitter.
British Prime Minister Theresa May has described it as a “terrible incident”.
“All my thoughts are with those who have been injured, their loved ones and the emergency services on the scene,” she said.
Labour leader Jeremy Corbyn said he was “totally shocked by the incident”.
“I’ve been in touch with the mosques, police and Islington council regarding the incident,” he said.
“My thoughts are with those and the community affected by this awful event.”
Police have sealed off adjacent streets, and London Ambulance Service said it has sent "a number of resources" to the scene.
The area was brimming with Muslim worshippers who had finished performing Tarawih, late-night prayers that are recited during the fasting month of Ramadan. The Muslim Council of Britain said its "prayers were with the victims."
“We are working closely with other members of the emergency services at the scene,” the ambulance service said.
“Our priority is to assess the level and nature of injuries and ensure those in the most need are treated first and taken to hospital."
Witnesses at the scene have described the incident as “horrible” and “hell”.
Monday's incident was the third since March involving a vehicle running into pedestrians in London. In March, a lone motorist mowed down dozens of pedestrians on Westminster Bridge, killing five, before exiting his vehicle and fatally stabbing a police officer.
Earlier this month, several people were arrested for an attack in which a van drove into pedestrians on London Bridge before assailants exited the vehicle and began stabbing passersby at Borough Market. Eight people died in the attack.
The UK is also reeling from a suicide bombing at Manchester Arena last month in which 22 people died and 119 were injured.
This story is developing.