The number of calls for help made by London's bus drivers has increased since under-16s were given free travel, the London Assembly Tories have said.
Tories say free travel for the under-18s is an affordable luxury
Figures obtained by them show from April 2003 to September 2005, drivers made 432 "code red" calls per month.
Following the introduction of free travel in September 2005 the average number of calls per month rose to 535.
Transport for London were approached by the BBC for a comment but failed to respond to the figures.
The Conservatives have said they got the figures following a meeting of TfL Surface Advisory Panel in January.
The statistics also show the incidents of code red calls - when the driver has to stop the bus and call for outside help - due to anti-social behaviour increased from 465 cases in September 2005 to 697 cases in October 2006.
Tory Roger Evans, deputy chairman of the London Assembly Transport Committee, said: "The mayor informs us on a regular basis that the introduction of free travel caused only a minor 'blip' in the number of offences committed on the bus network but never mentions how many code red calls are made by drivers due to their experiences of anti-social behaviour.
"These statistics show an upward trend in the number of incidents of anti-social behaviour on the bus network and this is a wholly unacceptable situation."