Dubbed "Eirene", after the ancient Greek Goddess of peace, the operation involves the biggest deployment of security forces in "Spain's recent history", according to the government.

A total of 10,000 agents backed by sniffer dogs and helicopters have been deployed to provide security for the 5,000 delegates attending the three-day summit, which gets under way later on Tuesday.

Spanish Defence Minister Margarita Robles said fighter jets and anti-aircraft artillery devices had also been placed on high alert to protect Spanish airspace.

US President Joe Biden and British Prime Minister Boris Johnson are among the leaders expected at the gathering of NATO members and a dozen specially invited nations.

"Madrid and Spain will be the centre of the world," Spanish Foreign Minister Jose Manuel Albares told private television Antena 3.

On Tuesday, police on foot and on horseback patrolled the streets of Madrid, which were monitored by police helicopters and drones.

The tightest security was around the IFEMA conference centre in the north-east of the capital, where the summit will take place.

Roads leading to the conference centre were cut off and the nearest metro station was closed.

Access to the hotels where delegations are staying was also restricted.

To avoid gridlock in the city of over three million, local authorities strongly recommended that people work from home if possible.

Madrid's Prado museum, which will host a gala dinner on Wednesday evening, will be closed to the public for two days.

The capital's imposing central square, the Plaza Mayor, will be closed from Tuesday afternoon and used as a parking space for delegate's vehicles.

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ANTENA 3 DE TELEVISION SA