A number of pedestrians have been taken to hospital after a bus mounted a pavement near Victoria station in central London.
Several people, including the driver of the route 24 bus, are understood to have been injured in the crash on Victoria Street on Thursday morning.
There are no reported fatalities, the Metropolitan police said.
No arrests have been made and an investigation has been launched, the force added.
Pictures from the scene show a number of emergency vehicles including police cars, ambulances and a fire engine. The road remains closed with all vehicles being diverted from the area, the Met said.
Emit Suker, 47, told PA news agency: “It [the bus] was coming from Westminster – it was going really fast and came off the road.
“There were about 15, 16 people inside the bus. People were screaming – it was terrible.”
Another witness told PA news agency: “I heard a massive crash – came outside and there was a woman on the floor with loads of people around her. Lots of people from the gym had run out to help her.”
A trail of diesel running down Allington Street has forced police to ban smoking in the area over fears of an incident.
A London ambulance service spokesperson said: “We were called at 8.20am today to reports of a road traffic collision on Victoria Street, Westminster.
“We have sent resources to the scene, including ambulance crews, advanced paramedics, a paramedic in a fast response car, a clinical team manager, an incident response officer and a command support vehicle. We’ve also dispatched London’s air ambulance.
“The incident is ongoing and we are working with our emergency services partners.”
Rosie Trew, TfL’s head of bus service delivery, said: “Our thoughts are with the people who have been injured following a bus incident at Victoria Street.
“We are working with the police and the operator, Transport UK, to urgently investigate this incident.
“This must have been a distressing incident for everyone involved and we have support available for anyone affected.”