By Steven Shukor
BBC News, London
Reported youth crime on buses has shot up by 55% since the introduction of free bus travel for under 16-year-olds.
Free travel for youngsters was introduced in 2005
There were 5,701 reports in the year since the scheme went live in September 2005 compared with 3,666 in the previous year, official figures show.
The Transport for London (TfL) figures were obtained by BBC London under the Freedom of Information Act.
However, TfL said the figures represent less than one crime report for every 250,000 journeys.
Observers say the figures, including allegations of vandalism, assault and theft, offer the first full picture of the impact the scheme has had on people's experience of bus travel in London.
TfL insists buses are "a low crime environment and a safe and reliable way to travel."
It said total crime allegations, including all age groups, had fallen by just under 4% from 40,854 in 2005/06 to 39,402 in 2006/07.
"Any crime is unacceptable, but the current level does not represent the very negative picture being painted by some opponents of free travel initiatives," said a TfL spokesman.
To address crime and anti-social behaviour, TfL said it was spending £70m a year to deploy 1,200 police community safety officers (PCSOs) on the bus network.
Angie Bray, leader of the London Assembly Conservatives said the figures confirmed there was a "growing problem" coinciding with the introduction of free bus travel.
"They ought to cause real pause and reconsideration as to whether this use of public transport is the best way to provide a free and safe travel for under-16s to school and back," she said.
"The figures are unacceptable as far as the travelling public are concerned.
"It's not just a problem for fare-paying passengers unable to get a seat, but there's also a real problem of threatening and intimidating behaviour on buses."
The Conservative group has called for the free bus travel scheme to be scrapped in favour of a dedicated school bus service.
Tom Scanlon, of the Transport & General Workers' (T&GW) Union, said youngsters had lost respect for bus drivers and fellow bus passengers.
"There is an assumption that because travel is free, they can get on and off an bus and they are answerable to no-one.
"It has an effect on ordinary passengers," he said.
"They just see groups of children going on and running wild. It can be disconcerting.
"We need more buses to cope with the increase in the number of youngsters travelling by bus."
He said the union has been sending drivers into schools to talk about bus travel with pupils in an attempt to encourage good behaviour.
"We explain to school children and young adults how their behaviour can affect the safety of other passengers.
"We support the introduction of free bus travel. The youth of today are the fare-paying adults of the future."
'Torrent of abuse'
Bus travellers Tom Wright and Valeria Martinelli set up a campaign calling for a ban on playing music on mobile phones on London buses.
"The reason we started the campaign is that we thought anti-social behaviour was getting worse and we were sick of it," said Mr Wright. "It's as if youngsters see buses as their own personal taxi service."
He said the couple's worst experience was when they asked a couple of teenage girls to lower their music.
"Instead of putting their music down, they responded with a torrent of abuse and sure enough the music volume went up," said Mr Wright.
"We alerted the bus driver who eventually managed to get the girls off. They followed us home for a while, which was quite intimidating."
Mr Wright suggested free bus travel should be limited to school hours.
"I would not like to see it scrapped completely because that would be a shame for the thousands of children who use the bus and don't create a fuss."