ROME – Italy’s new prime minister, Romano Prodi, pledged to pull Italian troops out of Iraq, saying yesterday that the war had been a “grave mistake” that could make “the whole region explode.”

“We consider the war and occupation in Iraq a grave mistake because it has not solved the problems of security, it has complicated them, and opened Pandora’s box,” he said, in his maiden speech to the senate. “Terrorism has found a new base and new excuses for internal and external terrorist action.”

Silvio Berlusconi, whom Mr. Prodi narrowly defeated in last month’s general election, sent 3,000 Italians to Iraq in 2003. The move, seen in Italy as a play for the affection of President Bush, was extremely unpopular. Almost 3 million people took to the streets of Rome to demonstrate against it.

In an attempt to win over the crowd before the general election, Mr. Berlusconi promised that troops would be home by last September, but he backtracked after he received a telephone call from Mr. Bush.

Mr. Prodi did not give a date for the pull-out, but said that he would negotiate with America on the issue.

Italy was a vital ally in Europe on the war in Iraq, and Mr. Prodi’s decision to withdraw troops leaves Prime Minister Blair isolated.

In the rest of his speech, Mr. Prodi promised a vigorous spring-clean that would wash away many of Mr. Berlusconi’s legacies. He said he would relax Italy’s stringent immigration laws, give new independence to the law courts, and draft a rigorous bill to prevent conflicts of interests.

The last item will particularly worry Mr. Berlusconi, since it may force him either to give up politics or his television and publishing empire. As a final blow to Mr. Berlusconi, Italy’s new transport minister, Alessandro Bianchi, vowed that the construction of the world’s largest suspension bridge, between Sicily and the mainland, would not go ahead. The bridge, Mr. Berlusconi’s most glittering vanity project, is “the most useless and harmful plan of the past 100 years,” said Mr. Bianchi.

The two-and-a-half mile bridge was expected to open in 2011. Mr. Berlusconi boasted that it would enrich Sicily by opening the island to more trade with the rest of Italy.

The former prime minister envisaged a double six-lane highway, with four further tracks for a high-speed railway line. The bridge’s supports would have been as high as the Eiffel Tower, and the equivalent of Italy’s entire annual consumption of concrete would have been poured into the waters off Messina.

Opponents, however, did not see the merit of spending billions on a suspension bridge in a notoriously earthquake-prone area when some parts of Sicily are still without running water.