SURFACE-TO-AIR missiles and between 1200 and 1500 soldiers and Royal Marines are being drafted in to bolster security at key Scottish sites during the G8 summit in July.

Special forces, including the SAS counter-revolutionary warfare squadron's sniper teams, are also likely to be deployed amid fears terrorists may try to infiltrate the 200,000 anti-capitalist protesters who are expected to converge on central Scotland from all over the world.

A quick-reaction flight of missile-armed Tornado F3 fighter aircraft is to be on 24-hour standby at RAF Leuchars to intercept and, if necessary, shoot down aircraft straying into an exclusion zone around the main conference site at Gleneagles Hotel.

Together with 10,000 civilian police – some from English forces – the military contingents will help guard heads of state such as George Bush, the US president, and Tony Blair, the prime minister, while the July 6-8 summit is held in the Perthshire hills.

The 550 marines of the fleet protection group have also been tasked with providing a final layer of defence against demonstrators who it is thought are planing to lay siege to the Royal Navy's Faslane submarine base and stage a separate protest against nuclear weapons as a diversion from the main event. The Clyde base is home to Britain's strategic deterrent Trident missile submarine flotilla and four nuclear-powered hunter-killer boats and is one of the most sensitive sites in the UK.

The three-day summit of the world's seven richest and most industrialised nations, plus Russia, is expected to be a magnet for international anarchist groups and economic demonstrators, prompting fears that terrorists could use the occasion as cover for assassination attempts on world leaders.

Air Force One, the American presidential jet, is to fly in to Edinburgh airport escorted by US fighters before moving to RAF Leuchars in Fife, where it will remain under heavy guard for the duration of Mr Bush's visit.

The original plan was to fly directly to the RAF base, but advisors now say this might have been regarded as provocative and "less than transparent", given the CIA's recent refuelling pitstops at Scottish airports while terrorist suspects were shuttled secretly round the world for interrogation.

US air force Galaxy transports are also due to deliver a fleet of armoured limousines to take the president and his party by road to Gleneagles. Sources say some will be decoys and that one may even be used to take America's first lady on a shopping trip.

The presidential party is to be allowed to deploy its own armed US secret service security detail as a prerequisite for Mr Bush's attendance.

Tayside Police, who are co-ordinating the overall security operation along with military and intelligence officials, have plans to cordon off Glasgow and Edinburgh and close the Forth and Kincardine bridges if necessary to forestall the violent demonstrations which have become the hallmark of previous G8 gatherings.

A number of extremist and anti-capitalist groups are known to have run training courses for a hardcore element of protesters who, intelligence sources say, will riot in an attempt to provoke security forces and guarantee global headlines. Faslane and the city centres of Glasgow and Edinburgh are known targets.

Although official sources refuse to comment on security arrangements, The Herald understands that soldiers from the Royal Scots and the Edinburgh-based Light Infantry have been requested to support the civil authorities.

All police leave across Scotland has been cancelled for the duration of the summit, and police riot teams have been honing their crowd-control skills on the site of the disused Law Hospital in Lanarkshire.

Edinburgh Sheriff Court is also understood to have cleared a two-week slot before, during and after the G8 to deal with trouble-makers.

Emergency holding pens at RAF Turnhouse on the outskirts of the capital are to be prepared to house large numbers of prisoners if arrests have to be made.

The Queen has switched the traditional early July investiture and garden party at Holyrood House to July 28 on the advice of the security services to avoid a clash with the summit.

Sources say that while contingency plans for every eventuality have been drawn up, the aim is to protect the vast majority of peaceful demonstrators and contain extremists with as little confrontation as possible.