Athens' Olympic security umbrella, including dozens of armed Patriot defence missiles and hundreds of surveillance cameras, started operating on Tuesday, as organisers prepare to safeguard next month's Games.

Almost 300 closed-circuit cameras were sweeping main avenues and squares while three police helicopters and a Zeppelin airship, equipped with more surveillance cameras, hovered above the capital.

Dozens of new Pac 3 (Patriot Advanced Capability) missiles were armed and in position at three locations around the capital, including the Tatoi military base near the athletes' Olympic Village, to provide a defence umbrella over Athens during the Games.

"This is the start of the operations. The system will gradually be operated in full," a police source told Reuters. "The helicopters and the Zeppelin will be flying almost around the clock from now until the end of the Games."

Army officials at the base said the Patriot defence missiles were locked into their launchers and ready to use against a potential threat from the air from now until the end of the Games on August 29.

"This particular squadron, along with other missile guided squadrons, is part of an anti-missile umbrella formed in the Athens region for the protection of the Olympic Games," squadron leader Lieutenant-Colonel Agamemnon Koliakos told Reuters Television.

Security forces on Tuesday also received 11 state-of-the-art surveillance vans, which will receive and monitor images from around the city. The coast guard will position six of them around the port of Piraeus, where seven luxury cruise ships to be used as hotels will berth during the Games.

Greece is putting in place the most expensive Olympic security plan ever, worth more than 1 billion euros ($1.22 billion).

By the August 13 opening ceremony, authorities will be deploying more than 70,000 security staff as well as thousands of cameras to secure the first summer Games since the Sept. 11, 2001, attacks on the United States.

NO THREATS

Greece has set up a seven-nation security advisory group, including France, Germany, Israel, Spain, the United States, Britain and Australia to provide intelligence and training, and has called on NATO for air and sea patrols.

But the government has assured there was no indication, or intelligence "chatter", of a potential attack in Greece during the Games, a statement backed by the international police organisation Interpol.

"I am not aware of anything like this," government spokesman Thodoris Rousopoulos said after story in The New York Times suggested Greece was concerned Muslim militants in the country could prepare for a strike during the Games.