The mayor said Londoners should be paid a "living wage"

The protest comes after mayor Ken Livingstone announced that the amount should be set as a minimum for those working and living in London.

The Rail and Maritime Union (RMT) says maintenance companies should set minimum standards of pay on subcontractors who employ the cleaners.

But the firms say they already work with subcontractors to ensure fair pay.

A spokesman for the mayor said: "The living wage should be paid by the maintenance companies, Metronet and Tubelines, to these employees from the huge profits they make each year from the Public Private Partnership.

"The cost should not fall on London taxpayers and passengers," he said.

'Absolute minimum'

General Secretary of the RMT Bob Crow said: "Low pay among Tube and rail cleaners is an absolute scandal and we insist on decent minimum employment standards.

"That means �7.20 an hour as the absolute minimum and a guarantee of 28 days' holiday."

A spokesman for Metronet, which is responsible for maintaining and upgrading parts of the Tube, said: "Metronet's policy is to see rates of pay gradually adjusted to reflect a closer alignment with the GLA's living wage.

"Cleaners are not our employees but we have been insisting on basic rates of pay when we renegotiate our contracts with the cleaning companies."