Twelve hundred more troops have been added to the defence of London's Olympics.
Aircraft that stray into restricted airspace could face "lethal force" if they fail to comply with directions from military aircraft.
The Royal Navy's largest warship, HMS Ocean, sailed up the Thames today as the military stepped up security preparations for the Olympics.
More than 120 personnel from the 4th Mechanized Brigade will parade through central London this afternoon.
The contingent have recently returned from a six-month tour of Afghanistan, where they were assisting the Afghan National Security Forces in Helmand Province.
The parade will leave Wellington Barracks, Westminster, at 3.20pm, and travel through the city to the Palace of Westminster. They will then attend a reception at the Houses of Parliament.
Parents of soldiers killed in Iraq and Afghanistan are to take part in protests up and down the country against plans to reduce the size of the army.
A protest is due to be held at the cenotaph in central London.
Demonstrations are also expected to be held at war memorials in cities including Glasgow, Leeds and Manchester on July 24th. The demonstrations have been organised by Dee Edwards, who set up the group Protest Against Ministry of Defence cuts.
On Thursday it was announced that the army will be cut from 102,000 to 82,000 personnel.
Dee said: "What the MoD are doing is completely wrong."What happens to all of these thousands of people when they are made redundant - how many more ex-soldiers are going to end up on the streets?
We will be holding the protests at 11am, the same time as Armistice Day is marked, to remember all the soldiers' lives who have been lost and to think about all the ones that could die in action in the future."
The Ministry of Defence says that similar missile defence systems have been deployed at all recent Olympic Games.
They will be in place by mid-July, along with Royal Navy helicopter carrier HMS Ocean (which will be moored in the River Thames), Royal Air Force Typhoon jets (which will be temporarily stationed at RAF Northolt), and Puma helicopters (at a Territorial Army centre in Ilford).
The Ministry of Defence has confirmed that ground based air defence systems will be deployed at the following sites in order to protect the Olympics.
Lexington Building, Fairfield Road, Bow, Tower Hamlets - High Velocity Missile
Fred Wigg Tower, Montague Road Estate, Waltham Forest - High Velocity Missile
Blackheath Common, Blackheath (Lewisham/Greenwich) - Rapier
William Girling Reservoir, Lea Valley Reservoir Chain, Enfield - Rapier
Oxleas Meadow, Shooters Hill, Greenwich/Woolwich - Rapier
Barn Hill, Netherhouse Farm, Epping Forest - Rapier.
Defence Secretary Philip Hammond MP said:
"Whilst there is no reported threat to the London Olympics, the public expects that we put in place a range of measures aimed at ensuring the safety and security of this once-in-a-generation event. Ground-based air defence systems will form just one part of a comprehensive, multi-layered air security plan which, I believe, will provide both reassurance and a powerful deterrent.
“We have undertaken a wide programme of engagement with the communities affected, involving relevant local authorities, landowners, MPs, Council Leaders, and community meetings. These have shown that, while people understandably have questions and concerns which we have sought to answer, broadly speaking communities are supportive of our work.
“A small number of activists object to the deployment of these defensive measures and a legal challenge to the Government’s decision to deploy GBAD has been initiated. The MOD will defend these proceedings vigorously and is confident of defeating them.”
The Ministry of Defence has confirmed that missile systems will be deployed at six sites in and around London for the duration of the Olympics.