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A giant clock counting down the days until the start of the London Olympics has been unveiled in Trafalgar Square, hours before tickets go on sale.

From Tuesday morning, 6.6 million Olympic tickets become available, 500 days before the Games begin.

People have a six-week window to apply for tickets on the London 2012 website.

The 6.5m (21ft) clock started counting down at 1930 GMT in the square, scene of great celebrations in 2005 when London won the bid to host the Games.

Champion rowers Pete Reed and Andy Hodge and sailors Iain Percy and Andrew Simpson pulled the covers off the clock, as 2012 chairman Seb Coe, London Mayor Boris Johnson and world champion heptathlete Jessica Ennis looked on.

Clad in brushed stainless steel panelling, it is two-sided with one side counting down to the Olympics and the other to the Paralympics.

Ms Ennis, who is aiming for her first Olympic gold at London 2012, said: "We all know that having a home Olympics is unique but the countdown clock is just bringing it all to light. It is all so exciting."

Lord Coe, winner of two Olympic golds, added: "Athletes' careers are based on timing and I hope that this milestone moment excites and inspires them to compete at the highest level in 2012."

People have until Tuesday 26 April to apply for tickets. Oversubscribed events will go to a ballot.

Several events will be free, such as the marathon, and 2.5 million tickets will be available for £20 and under. Others start at between £30 and £50.

Children under 17 will be able to "pay-your-age" to see some of the early heats, while the over-60s can watch for £16.

In total, there are 8.8 million tickets but some 1.2 million are reserved for various government bodies, the London mayor, sponsors and athletes.

Games organisers have faced calls from the London Assembly for a record of all tickets reserved for officials, politicians and VIPs to be published.

A further two million tickets for the Paralympic Games go on sale on 9 September.

There has been some criticism of the ticket-buying process as all online tickets can be paid for only with a Visa card.

Olympics organisers Locog say this is in recognition of Visa's sponsorship of the Games, but critics say that is unfair.

Visa Europe has said people who do not have a Visa debit or credit card and do not wish to get one can obtain a Visa prepaid card to purchase Olympic tickets.

Concerns have also been raised about payments made on Visa debit cards.

Locog says payment will be taken between 10 May and 10 June and people will be told by 24 June which events they have tickets for.

This could mean money going out of bank accounts before the buyer knows which tickets they are getting.

Efforts are being stepped up to curb ticket touting, with the government planning to raise the maximum penalty from £5,000 to £20,000.