In Rome, an initial 400 men and women were stationed at subway and railway stations and at an immigrant center, the Defense Ministry said. They were not expected to be deployed immediately in the capital's historic center.
In Milan, some 150 soldiers were patrolling the Duomo cathedral and sensitive sites such as the U.S. consulate and the city's synagogue. Soldiers were also deployed in Naples, Bologna and Palermo, Italian reports said.
More soldiers will be deployed in coming days, reaching an overall total of 3,000, said Lt. Col. Mario Busi of the Defense Ministry.
Premier Silvio Berlusconi's conservative government had proposed the measure as part of a security package that also toughens rules for illegal immigrants, whom some Italians link to crime. The measure, recently approved by parliament, remains in effect for six months.
Defense Minister Ignazio La Russa dismissed criticism that the soldiers would scare residents, saying that instead they could help address citizens' concerns on security. The people more likely to be afraid, he said, were "the thieves, the rapists, the criminals."
Berlusconi has made law and order a priority after winning April elections on promises to crack down on crime.
Some critics say the deployments will end up militarizing the cities, while others bring up the opposite argument that a deployment of 3,000 soldiers is little more than a show.
In the 1990s, soldiers were sent into Sicily to help free up police for investigations of the Mafia.
