Police officers from nine forces are also being drafted in to boost security at Olympic venues and athletes' hotels after G4S staff failed to appear.
The military may be asked to supply 2000 more personnel as already increasing numbers deployed rose to 17,000 after G4S failed to present the full quota of guards needed.
But the planning for more reinforcements has been hampered because G4S and Games organiser Locog have been unable so far to pinpoint where the latest shortfalls are.
Heathrow put half of its 1000 volunteers on duty and created special teams to deal with athletes' oversized gear.
Hundreds of agents were on the job to ease the long lines that have plagued the airport for months. Rows of VIP buses whisked teams and coaches to the athletes village in east London.
Motorists grappled with new traffic lanes painted with the Olympic rings and reserved for official vehicles and dignitaries.
Sailors were guarding the gates to Olympic Park.
The dispatch of Army officers to G4S headquarters follows a growing feeling in the Ministry of Defence that it would be necessary to set up a combined headquarters with the military taking a more active role in the overall command of the Games.
The Home Secretary, Theresa May, admitted yesterday that the Government still could not predict how many G4S guards would turn up for duty next week at the Olympic Park.
In her second emergency statement in four days on the crisis, she insisted ministers were alerted to it only last week. But London Mayor Boris Johnson contradicted her version of events.
General Sir Nick Parker heads the Standing Joint Command for the security contribution of the armed forces at present, with G4S running its own operational centre, which has seen the arrival of three more officers from the military in the past few days to work alongside others already there.
But with such large numbers of forces personnel now involved in the security operation, MoD officials believe there has to be closer integration. A senior Whitehall official said: "It's become obvious G4S not only needs help with the numbers but also administration. That's the reason a small number of extra military personnel have been sent in. This may have to be increased and there's a case for further integration when it comes to command and control."
G4S chief executive Nick Buckles was today to be pressed by the Commons' home affairs select committee on the extent of the security shortfall and why ministers were not alerted earlier to the looming crisis.
Asked in the Commons how many guards would be supplied by G4S, May said: "The precise balance of the number who will be provided will become clear over the next few days."
Yvette Cooper, the Shadow Home Secretary, said: "It is incomprehensible that with 11 days to go the Home Secretary still doesn't know how many staff G4S are likely to provide."
May repeated her insistence that the Home Office discovered only last week that G4S could not supply enough guards. But Johnson said it emerged six or seven months ago the "sheer volume of security, the need, was perhaps greater than expected ... Everybody organising the Olympics knew this was coming up ... ages ago".
Keith Vaz, chairman of the home affairs select committee, said: "This is the world's biggest security company - it's got 650,000 employees. They could have used some employees - 10,000 isn't a large proportion. Instead G4S choose to recruit from scratch for the greatest show on earth."
His committee is to summon May to give evidence about the debacle, as well as Office for Security and Counter Terrorism head Charles Farr, who runs the Games' anti-terror strategy.
Security sources say although G4S is getting pilloried for the security woes, part of the problem was caused by a Cabinet committee headed by David Cameron approving a 500 per cent rise in security guards from the original total of 2000 in December.
"G4S saw more pound signs and Locog didn't scrutinise at all the process G4S had in place for all this," said one official.
- Independent, AP
