British police shot dead a man on London Bridge on Friday, Sky news reported.
The BBC said two shots had been fired. Videos and photographs on Twitter showed several police cars and buses on the bridge and a truck straddling several lanes.
"We are in the early stages of dealing with an incident at London Bridge," police said in a statement.
A spokesman said: "It appears somebody has been shot."
The ambulance service said it had crews at the scene.
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.
Earlier this month, Britain lowered its national terrorism threat level to "substantial" from "severe," its lowest level since 2014.