Sadiq Khan has called for an urgent inquiry into the death of a contractor who was working on a travelator at Waterloo station early this morning.
The engineer had been on the moving walkway connecting the Jubilee line and the Northern and Bakerloo lines.
Passengers were being told to avoid the interchange where possible, but the station has remained open.
British Transport Police are investigating the death, which is not thought to be suspicious. It is not yet clear how the man died.
The Mayor of London said on Twitter: "Very sad to hear of the tragic death of a contractor at Waterloo Station this morning. I know I speak for everyone at City Hall and TfL in sending our deepest condolences to their family & friends."
Mr Khan said an urgent inquiry into what happened is "vital" to ensure it cannot happen again.
At 5.24am Transport for London wrote on Twitter: "Waterloo Station: No interchange between lines, due to a fault with the the travellator. Please exit the station and walk back in."
A tweet by Jubilee Line just after 7.30am read: "Waterloo Station - A one way system has been implemented at this station due to an ongoing police investigation, customers are advised to avoid interchange where possible."
Waterloo is the largest, and one of the busiest, stations in London.
Detective Inspector Darren Gough said: "This is a truly tragic incident, and our deepest condolences are with the man's family.
"They are currently being supported by specially-trained family liaison officers as they come to terms with this devastating news.
"We are now working closely with the Office for Road and Rail (ORR) to conduct a thorough investigation into the incident and establish exactly what happened to lead to the man's very sad death."
British Transport Police said in a statement: "Officers were called just after 2.20am this morning (Wednesday 18 September 2019) to London Waterloo Underground Station following reports that a contract worker had received an injury.
"London Ambulance Service and London Fire Brigade were already on scene and it became apparent the incident was more serious.
"Lifesaving attempts were very sadly unsuccessful and a man was pronounced dead at the scene.
"Officers are currently working to identify the man and inform his family. Enquiries into the incident continue."
A Transport for London official said: "We are working with colleagues in the British Transport Police following an incident at Waterloo station in the early hours of this morning. An investigation is under way."