An electric double-decker bus caught fire during the morning rush hour in south-west London.
The Met Police said emergency services were called about the incident on Wimbledon Hill Road/Alwyne Road in Wimbledon shortly after 07:20 GMT.
No injuries have been reported but a critical incident was declared, police said.
Max Pashley, a local resident, told City A.M.: "We heard a huge bang. We were terrified."
Road closures and cordons are in place and are expected to be for some time.
A Met Police spokesperson said: "We thank the local community for their patience and co-operation as emergency services work at the scene."
People have been advised to avoid the area.