Boris Johnson said London needs more police officers on the beat Boris Johnson

The Conservative MP unveiled his plans to fight "incivility, rowdiness, violence" on public transport.

He said youngsters who had their Oyster cards taken away would have to do community service to get them back.

He also wants Police Community Support Officers (PCSOs) to do more paperwork to help get more police on the beat.

Mr Johnson said young people would value their free Oyster cards if there was a risk of losing them.

If I think to when I was a kid growing up on the streets of Camden it's a lot rougher now or a lot scarier Boris Johnson

In an interview on BBC One's Andrew Marr Show, he said: "If people knew that systematically they were going to be deprived of this then they would be much less willing to intimidate other passengers.

"Kids who want to get back the privilege of free travel have to do some kind of community service, voluntary service, graffiti removal painting a community centre shed, whatever it happens to be."

The Henley MP said crime would be at the "top of his priorities" if he became mayor.

Mr Johnson added: "If I think to when I was a kid growing up on the streets of Camden it's a lot rougher now or a lot scarier, put it that way, than it was 35 years ago.

"I think there are things that we can do about that. The first and most obvious thing is to get more police on the beat."

He suggested that PCSOs could be "in the back room" working on administrative duties, which would "liberate" frontline officers.

A spokesperson for Ken Livingstone's mayoral campaign said free travel cards are already confiscated for anti-social behaviour on buses.

She added: "Anyone cautioned by the police or convicted of a criminal offence loses their card permanently, which is a very effective deterrent, whereas Boris Johnson's proposal would lessen this punishment."