LONDON — At least seven people were killed and dozens more were injured when a tram derailed near London on Wednesday, the British police and emergency services said, and the driver of the tram has been arrested.

The police were called to the scene in Croydon, south of the British capital, shortly after 6 a.m., and officers, firefighters and paramedics worked through the morning to free people who were trapped inside the tram, the British Transport Police said in a statement.

“At least seven people are now confirmed as having died as a result of the incident,” the transport police said in a statement. The cause of the accident was unclear, the police said, and an investigation was opened.

Several of the injured were treated at the scene and 51 were taken to the hospital, Peter McKenna, the deputy director of operations for the London Ambulance Service, said in a statement.