PRIME MINISTER RETURNS TO LONDON
The ambulance service declared a "major incident" in the area and London Bridge station, a busy commuter hub, was closed.
People in buildings around the scene were evacuated to the north side of the River Thames by security services.
Prime Minister Boris Johnson was on his way back to Downing Street and Basu said he would be giving him a full briefing.
"I'm being kept updated on the incident at London Bridge and want to thank the police and all emergency services for their immediate response," Johnson said in a statement.
London Bridge was the scene of an attack in June 2017 when three militants drove a van into pedestrians and then attacked people in the surrounding area, killing eight people and injuring at least 48.
Islamic State said its fighters were responsible.
The month before, a suicide bomber killed 22 children and adults and wounded 59 at a packed concert hall in the English city of Manchester, as crowds began leaving a concert by U.S. singer Ariana Grande.
And in March the same year, an attacker stabbed a policeman close to London's parliament buildings after a car ploughed into pedestrians on nearby Westminster Bridge.
Six people died, including the assailant and the policeman he stabbed, and at least 20 were injured in what police called a "marauding terrorist attack".
Earlier this month, Britain had lowered its national terrorism threat level to "substantial" from "severe", its lowest level since 2014.