The main openening ceremony doesn't take place until Friday in the Olympic stadium but with two days to go football steals the limelight.
Despite fears over the ability of London's infrascruture to cope with olympic traffic, in Cardiff Welsh Secretary Cheryl Gillan said she was "immensely proud that the world's greatest sporting event starts in Wales."
Culture Secretary Jeremy Hunt will join Gillan in the Welsh capital for the first sporting event of London 2012.
Every very goal scored by Britain's men's and women's football teams in the Olympic group stages will see 50 footballs donated to charity.
Wembley Stadium, a venue for both the London 2012 and London 1948 Games, is making the gesture as part of the Football Association's Olympic legacy plans.