This comes a day after the government announced a series of measures aimed at cracking down on "extremism".
Police and Carabinieri officers backed by Italy's special anti-terrorism unit, the DIGOS, searched homes and buildings in Turin, Florence, Bologna, Rome, Naples and other cities from early morning on Wednesday.
Hundreds of people underwent identity checks, and documents and computer equipment were seized for analysis.
Searches also took place in North African communities in Siracusa and Ragusa on Sicily's east coast.
The authorities had not announced any arrests several hours into the operation, which was still ongoing at midday.
However, the operation was clearly aimed at verifying the movements of suspected extremists and sweeping Muslim communities for information and material which may prove useful as Italy upgrades its effort to prevent a London-style attack.
In the aftermath of the attacks, Prime Minister Silvio Berlusconi said Italy was in "the firing line" of terrorism in Europe because of its support for the US-led operation in Iraq.
The measures announced on Tuesday in parliament by Interior Minister Giuseppe Pisanu include closer monitoring of suspects, doubling from 12 to 24 hours the length of time a suspect can be held without being charged, questioning of terrorism suspects without the presence of lawyers and easier access to mobile phone and Internet records.
Under new measures, suspects
can be held without charge
Surveillance is also expected to be stepped up around mosques.
Police are expected to provide details of the ongoing search operation later on Wednesday.
Rome mayor Walter Veltroni said he shared the interior minister's concern over the threat of terrorism.
"I have said before, after the attacks in New York and Madrid, that nobody should underestimate the threat faced by all European countries," Veltroni said.
In keeping with the heightened sense of threat in Italy, which has already doubled security patrols in and around its main airports and railway stations, the head of the military security intelligence service SISMI, Nicolo Pollari, is to address parliament's security committee on Thursday on the intelligence services' response.
In a separate development, a court in the northern city of Brescia on Wednesday jailed two North African men suspected of being members of a terrorist cell in Cremona outside Milan.
An imam who preached at the Cremona mosque, Muhammad Rafiq, was sentenced to four years and eight months. The second man, Kamal Hamroui, was sentenced to three years and four months.
A third man, Najib Rouass, was sentenced to one year and four months in jail on a minor charge.
The convictions were the first in Italy under charges of international terrorism, a law introduced in the wake of the 11 September 2001 attacks on the United States.