A van struck worshipers leaving a north London mosque early Monday morning, causing several casualties, local media reported.
Metropolitan Police said they were dealing with a "major incident." A Sky News producer at the scene said the crash took place shortly after midnight local time as worshipers were leaving the Finsbury Park Mosque at midnight Ramadan prayers.
The Guardian reported that 10 people were hurt, three of them seriously.
Police arrested the van's driver, but were trying to determine whether the collision was accidental or deliberate.
"We have been informed that a van has run over worshipers as they left #FinsburyPark Mosque," the Muslim Council of Britain tweeted early Monday. "Our prayers are with the victims."
Locals massing at barriers, angry at police, people confused about what has happened. pic.twitter.com/VCN9Y3PTVb— Brian Whelan (@brianwhelanhack) June 19, 2017
One eyewitness who lives nearby told the BBC she saw people "shouting and screaming."
Cynthia Vanzella, another witness, wrote on Twitter: "Horrible to watch police officers doing cardiac massage at people on the floor, desperately trying to save them. I just hope they did."
The Finsbury Park mosque was associated with extremist ideology for several years after the 9/11 attacks in the United States but was shut down and reorganized. It has not been associated with radical views for more than a decade.
Britain's terrorist alert is set at "severe," meaning an attack is highly likely, Sky News reported.
The incident comes weeks after a van hit pedestrians on London Bridge and knife-wielding attackers killed eight people nearby. Police killed the three attackers.
Contributing: The Associated Press. Follow Greg Toppo on Twitter: @gtoppo