Official results by the Interior Ministry showed Prodi had won four of the six seats in the Senate elected by Italians living abroad, giving him the margin he needed to win both houses of Italy's parliament.

In the 315-member Senate, official returns showed Prodi had won 158 seats to 156 by the center-right, with one independent.

In the lower house, Prodi's coalition won 49.8 percent of the vote to Berlusconi's 49.7 percent. However, thanks to a new, fully proportional electoral system pushed through by the conservatives against the center-left's opposition, the winning coalition in the lower house gets at least 340 deputies, or 55 percent of seats, regardless of the margin of its victory.

But with Prodi's coalition winning the lower house by about 25,000 out of the 38 million votes cast, Berlusconi called for a recount. He said there were also many irregularities in the vote abroad, making it "possible that this is not a vote we can say is valid."

"We won't hesitate to recognize the political victory for our adversaries, but only once the necessary legal verification procedures have been completed," he said, adding that he had full confidence in the checks.

After a night of swinging projections and with a dramatic vote count still under way, Prodi vowed he would form a strong government in a country mired in zero-percent economic growth and almost evenly split by the vote.

But Prodi would preside over a potentially unwieldy coalition, in addition to a weak popular mandate. The center-left, while built on two mainstream parties, includes a mixed group of smaller formations ranging from Catholics to communists.

With a high voter turnout of about 84 percent, analysts talked of a deeply split country, and expressed uncertainty over what might happen next.

"Italy is a country that has been divided since 1948," said analyst Edoardo Novelli, referring to the landmark Italian election between the U.S.- and Vatican-backed Christian Democrats and the Soviet-supported Communists.

"But it has never been so on the line," added the professor of political communications at Milan University.

Coming back from a consistent gap in the polls before the election, Berlusconi's forces gained around half of the popular vote.

The premier suggested that should the two houses be divided, Italy could follow Germany's model and create a "grand coalition" between the left and right.

"I think that we maybe need to take the example of another European country, perhaps like Germany, to see if there's not a case to unify our forces to govern in agreement," he said.

Prodi quickly dismissed the suggestion.

"We went before voters with a precise coalition and the electoral law assigned us a number of lawmakers in the Chamber and in the Senate that allows us to govern," he said.

Berlusconi became Italy's longest serving premier since World War II thanks to his five years in power. Despite a tumultuous tenure, including the loss of some ministers along the way and a Cabinet reshuffle last year, the conservative media mogul delivered a sense of stability to the country.

The Interior Ministry stressed Tuesday that the results must still be confirmed by Italy's highest court, and that parliament's election committees would have to rule on any challenges.

A new government would then have to win a vote of confidence in both houses of parliament, which under the Italian system have equal powers and largely duplicate work.