British police sealed off Westminster Bridge in central London next to parliament on Thursday following reports of an unattended vehicle.
Officers initially blocked vehicles and pedestrians from the crossing, then let pedestrians through again soon after.
"Officers are in attendance," a police spokesman said.
Britain is currently on its highest alert level of critical meaning an attack by militants might be imminent following the suicide bomb attack in Manchester on Monday.
In March, a man killed five people after driving into a crowd of pedestrians on Westminster Bridge in central London before stabbing a police officer in the grounds of parliament.