The leaning tower of Pisa has defied gravity for more than 800 yearss - but now Italian authorities are afraid it might be knocked down in a terror attack.

The first of several gates aimed at protecting the tower as well as the adjacent medieval cathedral and baptistery has been put into place.

The tower scores high on a list of monuments and buildings at risk, drawn up by security chiefs last year.

The 250,000-euro project will install two more iron gates and a CCTV system.

But the regional superintendent for the artistic and historical heritage of Tuscany, Antonio Paolucci, did not seem to impressed.

"If al-Qaeda want to carry out an attack, they will go ahead with it. It won't be two things put up in front of it [the tower] that will stop them," he said.

The leaning tower was reportedly one of the targets of a string of bombings carried out by the Sicilian Mafia in 1993 though it was not hit.

In an unprecedented attack on Italy's cultural heritage, Cosa Nostra planted bombs at the Uffizi Gallery in Florence as well as near two churches in Rome.

But in December 2003, Italian intelligence revealed that the tower was in the sights of Islamist terror groups.

The attack was not deemed very likely, but authorities decided to put up barriers that would protect the monuments from a possible car bomb attack.

The 13th-Century tower is one of Italy's top tourist attractions.

It was closed in 1990 when experts said it risked toppling over.

The million euro restoration works corrected the lean by 45 centimetres, guaranteeing the tower's survival for two more centuries at least - provided it does not become a terrorist target.

(Agencies)