About 700 cleaners who work on the Tube have walked out in the first of a series of strikes to demand higher pay and better working conditions.
The Rail, Maritime and Transport (RMT) union members want a London "living wage" of £7.20 an hour instead of the current £5.50.
They began the 24-hour strike at 1850 BST on Thursday and plan to walk out again for 48 hours on 1 July.
Transport for London (TfL) said the strikes were "completely unnecessary".
A TfL spokesman said: "Following the transfer of Metronet to Transport for London, we will be working with Metronet and its sub-contractors to ensure that they pay their employees who work on the Tube the London living wage.
"We have already reassured all interested parties, notably the trade unions, that we are taking this commitment forward.''
But RMT leader Bob Crow described the current pay and conditions as "outrageous" and said cleaners were not entitled to receive sick pay, travel facilities or a decent pension.
"Our members have been subjected to massive intimidation but tonight they will make history by striking for the first time ever for the living wage their fat cat employers have so far denied them," he said.
The RMT also called for an end to the practice of cleaners being fired from their jobs without a disciplinary hearing or right to appeal against the decision.
An early day motion supporting the cleaners was tabled by Hayes and Harlington MP John McDonnell and backed by 24 other London MPs.
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