British Prime Minister Theresa May said on Sunday that the incident in London was being treated as a potential act of terrorism.

"Following updates from police and security officials, I can confirm that the terrible incident in London is being treated as a potential act of terrorism," May said just over four days before voting is due to begin in a June 8 national election.

"This is a fast moving investigation," May said. "I want to express my huge gratitude to the police and emergency services who are on the scene. Our thoughts are with those who are caught up in these dreadful events."