Cleaners on London Underground have mounted picket lines after walking out on strike in a dispute over an Olympic bonus payment.
Members of the Rail Maritime and Transport union employed by private contractors are taking industrial action for 48 hours.
LU and employers Initial said services were not being affected by the stoppage.
An Initial spokesman said: "Service is not being impacted with the vast majority of our colleagues continuing to do an excellent job and working as normal.
"Initial Facilities is proud to support its colleagues on the London Underground contract by paying the London Living Wage. More recently, we have also organised free Underground travel for colleagues during their shift throughout the Games period and launched a new recognition programme.
"Our colleagues had pay increases consistent with the London Living Wage in September 2010 - 3% - and September 2011 - 5%. Our next annual pay review discussions are due later in the year and we have no plans to change this process which applies to all colleagues and is consistent with previous years."
RMT general secretary Bob Crow said: "With the eyes of the world on the London Olympics the many millions enjoying the events around the globe must not forget the London transport cleaners on minimum pay rates doing some of the dirtiest jobs in often appalling conditions with no Olympics recognition and reward whatsoever.
"It is these same staff who have played a key role in keeping services clean and safe who are now forced again by their employers to take action for a living wage and recognition of their Olympics efforts."
Phil Hufton, asset performance director for London Underground, said: "This industrial dispute is between Initial and its staff. We have reviewed our staffing plans and have already put contingencies in place. We anticipate no impact on London Underground services as a result of this industrial action."
Initial said workers who voted in favour of action represented around 10% of a team of over 1,500 people working on the London Underground contract in a range of roles, including cleaning, vegetation clearing, tree surgery, fencing, administration and supervision.