London Mayor Sadiq Khan called the incident "a horrific terrorist attack" in a post on Facebook on Monday morning.
"We don't yet know the full details, but this was clearly a deliberate attack on innocent Londoners, many of whom were finishing prayers during the holy month of Ramadan," he said.
"While this appears to be an attack on a particular community, like the terrible attacks in Manchester, Westminster and London Bridge it is also an assault on all our shared values of tolerance, freedom and respect."
Speaking on Monday morning the Metropolitan police's deputy assistant commissioner, Neil Basu, urged the public to remain "calm and vigilant."
"Now is a time, once again, for Londoners to stand together to face those who seek to divide us," Basu said.
Additional police have been deployed in all London boroughs to reassure communities, particularly those observing the Muslim festival of Ramadam.
"This was quite clearly an attack on Muslims who looked like they were probably Muslims and they were coming from a prayer meeting," Metropolitan Police Commissioner Cressida Dick said during an update on Monday afternoon.
"We treat this as a terrorist attack and we in the (Metropolitan Police) are as shocked as anybody in this local community or across the country at what has happened."
The incident follows a series of attacks in Britain. Eight people were killed and 50 injured on June 3 when three Islamist militants drove into pedestrians on London Bridge and stabbed people at nearby restaurants and bars.
On March 22, a man drove a rented car into pedestrians on Westminster Bridge in London and stabbed a policeman to death before being shot dead. His attack killed five people. And on May 22, a suicide bomber killed 22 people at a concert by American pop singer Ariana Grande in Manchester in northern England.
The attack also comes days after the one year anniversary of Jo Cox's death, a member of parliament who was murdered in her constituency in West Yorkshire by a man known for his association with far-right organizations.
—Reuters contributed to this report.