LONDON » London marks the two-year countdown to the 2012 Summer Games today by throwing open venues to star athletes, issuing a call for 70,000 volunteers and opening the first official Olympic store for sale of mascots and other souvenirs.

A series of activities were planned across the city to celebrate the buildup to the opening ceremony on July 27, 2012, when London will welcome the world to the greatest sports show on earth.

The spotlight will be on the Olympic Park site in east London, where athletes will test out some of the key facilities that are still under construction but well on track to be completed next year.

Michael Johnson, a former Olympic 200- and 400-meter champion from the United States, will race against youngsters on a specially laid track in the 80,000-capacity main stadium. British cyclist Chris Hoy, a four-time gold medalist, will be the first to try out the velodrome. Former NBA player John Amaechi will shoot a few hoops at the basketball arena.

"We’re not just the next games," organizing committee president Sebastian Coe said in an interview with the Associated Press. "We’re the next big global event. The World Cup is out of the way. That’s where the world is coming. There’s nothing else between us and the Games."

Organizers were launching the Olympic volunteer program and urging people to apply for specialist positions such as doctors, anti-doping personnel and scoreboard operators. The signup for more general volunteer positions will open Sept. 15.

"Today we enter another new phase of London 2012—bringing the Great British public on-board," sports and Olympics minister Hugh Robertson said. "Volunteers are the heartbeat of every Olympic and Paralympic Games and are crucial to their success. We need skilled and passionate people who have the commitment to make 2012 the best Olympic and Paralympic games ever."

Mayor Boris Johnson was announcing a separate program to recruit 8,000 "London Ambassadors" who will man key points across the city, such as railway stations and tourist spots, to help guide visitors around the capital.

Organizers also urged fans to keep registering their interest in tickets, which will go on sale next year. So far, more than 1.4 million have registered.