The union is seeking a £500 bonus for members who work during the Olympic Games.
Staff working on the DLR, Tube and London Overground have all agreed extra Olympic payments.
Although London’s bus services are regulated by TfL the drivers and other staff are employed by private bus companies.
Mayor Boris Johnson has secured £8.3m from the Olympic Delivery Authority to settle pay claims and has called on employers and the union to settle their dispute. However on Thursday conciliation talks between the parties broke down without agreement.
Operators Arriva, Metroline and Go Ahead last night won a High Court injunction blocking their workers from striking. Staff employed by other companies started their walkout at 03:00 BST Friday.
Transport for London say “over a third of bus services” are running this morning and are urging Londoners to use other transport methods and to use its travel information services to plan their journeys.
Transport Commissioner Peter Hendy said: “It is now clear that the leadership of Unite were intent on a strike all along. They have pursued this unnecessary course of action despite an extra £8.3m being brokered by the Mayor that would allow every bus driver in London in a garage where one or more routes were affected by the 2012 Games to gain, over the 29 days of the competitions, about £500.
“Despite additional offers to supplement this from the bus companies the Unite leadership have refused to defer the strike to give time for further negotiations or for any of the offers to be put to their members. I’m sorry that Londoners are therefore going to be disrupted today.
“We are now working together with our partners to ensure that Londoners and visitors have alternative travel options and the necessary advice to enable them to get around. I would advise everybody to use other transport services, to consider walking and cycling for shorter journeys and to check before travelling at www.tfl.gov.uk/buses”
