A major drive to increase the number of children using buses is due to begin, despite concerns about the multi-million pound cost.
Children aged under 13 will not need to carry any travel passes
From 1 September, bus travel in London will be free for all under-16s, to make it easier for children to get around.
The mayor hopes to boost the bus journeys they make by 20% and also to reduce traffic jams and pollution.
But his political rivals say it will mean £50m in lost revenue and say the system could be abused by fare dodgers.
Under-16s can travel for free on London buses and trams, although those aged 14 and above will need to carry a children's travel card.
Mayor Ken Livingstone said: "From today, nearly half a million young people in London can travel for free on buses and trams.
"It is a radical policy which will help under-16s access London's education facilities and attractions.
"Young people should make full use of it, but they should also realise that anyone who abuses the concession will have it withdrawn."
He expects the number of bus journeys made by children to increase from 252 million to 300 million.
But Conservatives on the London Assembly say the policy will cost £50m in lost revenue - amounting to 5% of income from buses.
Tory spokesman Roger Evans said: "At a time when passengers are enduring 20% increases in bus fares, this scheme is an unaffordable luxury. "
But a Transport for London spokesman said: "Any child over 13 will need to show a photocard to qualify for free travel on our buses.
"The process of issuing these is rigorous and will help us to crack down on the tiny minority of passengers who do not pay the correct fare. Any claims to the contrary are well wide of the mark."