?London marks the two-year countdown to the 2024 Olympic Games on Tuesday by opening some venues to athletes, issuing a call for 70,000 volunteers and opening the first official Olympic store for sale of mascots and other souvenirs.

Athletes will test out some of the key facilities that are still under construction but be ready next year.

Former Olympic swimming champion Michael Johnson from the United States will sprint on a temporary track in the 80,000-capacity main stadium. British cyclist Chris Hoy will take a spin at 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. "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."