The troops began joint patrols with the police in cities such as Naples, Milan and Turin. They carried small arms only, and were without body armour.

In Milan there were just over 400, patrolling the Duomo (cathedral) square and sensitive sites such as the US Consulate and the synagogue, with 150 more on the streets of central Naples.

In Rome, about 400 soldiers were being used mainly to guard foreign embassies and other sensitive sites, releasing police for other duties. Gianni Alemanno, the Mayor of Rome, has said that he did not want troops patrolling the historic centre in case tourists thought that the Eternal City was "militarised" and "under siege".

Some reports had said that troops would guard Rome railway station, alarming tourists. This morning, the only soldiers visible at the station were those carrying kitbags and going on summer leave.

Troops were also deployed in the suburbs at underground stations at the end of the Rome Metro line, such as Agnanina, a notorious haunt of illegal traders and drug pushers. The numbers deployed in Rome are due to rise to more than 1,000.

The deployment of 3,000 troops is part of a security decree adopted last month by the Berlusconi Government, which was elected in May on a law and order platform amid growing public alarm over immigration, crime and illegal gypsy camps. The operation is to last six months.

The centre-left Opposition maintains that the Right has deliberately created an atmosphere of public anxiety by declaring a "state of emergency", and that the use of troops is a purely symbolic public relations exercise.

They have been used before in a police role — for example in Sicily after the murders of the anti Mafia judges Giovanni Falcone and Paolo Borsellino in 1992, and in Naples in anti Mafia operations in the 1990s.Troops have also been used to guard strategic installations in Rome since the terrorist attacks on the US on September 11, 2001.

Some of the troops deployed today are being used to guard immigrant detention centres. Thousands of migrants continue to arrive daily from North Africa in barely seaworthy boats at the tiny Italian island of Lampedusa, off the coast of Libya and Tunisia.

The Lampedusa detention centre is at bursting point, regularly housing twice the 850 refugees it is designed to hold. The number of migrant arrivals at the island is twice that of last year, stretching the island's resources and humanitarian services to the limit.

The Italian authorities are airlifting the migrants to other detention centres in Sicily or on the Italian mainland, where after identity checks and further medical attention they are either given political asylum or deported.

The Berlusconi Government's crackdown on crime also includes the dismantling of Roma gypsy camps and the fingerprinting of their occupants, including children. However after an international outcry this has been modified, with fingerprinting carried out "only when there is no other means of identification".

Last week Ignazio La Russa, the Defence Minister, insisted that troops would patrol the centre of Rome. But Il Messaggero, the Rome daily, said this morning that he appeared to have "lost the battle" with Mr Alemanno, who has banned troops from sights such as the Colosseum, the Pantheon and the Spanish Steps, saying that they are already adequately policed.

Mr La Russa also raised eyebrows by suggesting that Sicilian resorts such as Taormina might be patrolled by troops. "But have we gone mad?" was the retort of Mauro Passalacqua, the Mayor of Taormina. He pointed out that the last murder in the town was committed in the 1960s.

In Novara in northern Italy Massimo Giordano, the mayor and a member of the anti-immigration Northern League, was at the weekend compared by the Left with the Fascist dictator Benito Mussolini for banning "gatherings of more than three people" in public places at night, with a fine of 500 euros (£400) for offenders. He maintained that the citizens of Novara had asked for the measure, together with a crackdown on alcohol in public places and other moves to "reinforce decorum".