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A bike hire scheme designed to encourage thousands more cycle journeys in central London has begun.

So far, more than 12,000 people have signed up to the idea, far outstripping the 5,000 bicycles which will be available at special docking stations.

Transport for London (TfL) has admitted it had been expecting "teething problems".

But it insisted that although members outnumbered the number of bikes, people would not face a shortage.

Already, more than 12,450 keys have been handed out to Londoners enabling them to unlock bikes left at 315 docking points across the city.

The keys cost £3 and the cost of using the cycles varies from £1 for an hour to £50 for 24 hours. TfL and operator Serco expect to roll out the scheme to casual users after a month.

About 12,000 people had signed up to the scheme - with some asking for more than one key.

But only 6,000 keys had been activated. Until they are activated, cyclists will not be able to withdraw the bikes from docking stations.

A TfL spokesman said: "We are expecting there will be some teething problems with (the) scheme when it starts.

"This could include some technical issues and we have yet to learn the pattern of how people use the scheme."

The 23kg bikes with three gears do not come with locks and TfL said this was done to deter people from keeping the cycles for long periods.

They are automatically locked at the docking stations.

Docking stations will be available in Camden, City of London, Hackney, Islington, Lambeth, Kensington and Chelsea, Southwark, Tower Hamlets, Westminster and several of the Royal Parks.

London Mayor Boris Johnson said London had been "filled with thousands of gleaming machines that will transform the look and feel of our streets and become as commonplace on our roads as black cabs and red buses".

"My crusade for the capital to become the greatest big cycling city in the world has taken a gigantic pedal-powered push forwards," he added.

London Assembly Green Party member Jenny Jones said: "The bikes should have locks and bigger baskets. A target date should be set for people to be able to use Oyster (travel) cards to pay for the bike hire.

"The scheme also needs monitoring closely to ensure that the cost is not putting low-income Londoners off using the bikes."

TfL is hoping to reach its target of 6,000 cycles and 400 docking stations later this year.

Already anti-war activists have placed large stickers about the conflict in Afghanistan on the back of some bikes.

Other demonstrators left stickers on bikes at Hyde Park Corner in protest at Barclays' sponsorship of the scheme, unhappy about the bank's record of investing in defence companies.

A man was arrested for allegedly causing criminal damage by kicking a bike at a docking station in New Inn Yard, Shoreditch, east London.

The 23-year-old was bailed until next month after being detained at 0230 BST.