By GINGER THOMPSON

PORT-AU-PRINCE, Feb. 16 — The protests that paralyzed cities across the country turned into celebrations on Thursday as news spread that René Préval, a former president with overwhelming support among this country's poor, had been declared the next president.

Even as the news spread across the capital, Mr. Préval withdrew into the silence that characterized his past leadership, canceling a news conference and staying inside his de facto headquarters in his sister's house. And questions intensified over how he would resolve the country's many troubles and whether he would bring his old mentor, former President Jean-Bertrand Aristide, back out of exile.

In a hastily arranged news conference at 3 a.m., the members of Haiti's Provisional Electoral Council officially announced an agreement to change the way they had tabulated the results from last week's elections, giving Mr. Préval the 50 percent vote total he needed to become president without a run-off.

The agreement, forged during marathon negotiations on Wednesday between the council, Haiti's interim government and Mr. Préval's Lespwa political party, came after Mr. Préval challenged results showing him below the 50 percent mark.

Mr. Préval had charged that the elections were rigged against him, and throngs of his supporters paralyzed cities across the country with barricades of burning tires, stirring fears that the whole country might go up in smoke.

Foreign diplomats rushed to answer questions about the legitimacy of the back-room negotiations that brought Mr. Préval to power, trying to save the credibility of an election they consider crucial to stabilizing the troubled country. The diplomats praised the process as respectful of the law and of the votes cast by the majority of the Haitian electorate.

Speaking at a Congressional budget hearing on Thursday, Secretary of State Condoleezza Rice hailed the announcement that Mr. Préval would be president. "We are going to work with the Préval government," she said. "We want this government to succeed."

Mr. Préval, a 63-year-old agronomist, was president from 1996 to 2001 and is a protégé of Mr. Aristide, who was ousted two years ago by a violent uprising. As the recent election proved, Mr. Préval still commands overwhelming support from the same poor masses that hailed Mr. Aristide.

Now, the biggest challenge Mr. Préval faces as Haiti's president played out across the capital on Thursday, illustrated in the wide and hostile gap between his supporters and his critics, including many among the country's wealthy minority, who have charged Mr. Préval with resorting to force to bully his way to power.

Supporters of Mr. Préval began to gather at sunrise in front of the national palace. Many, like Marie Suze, 30, said they had not slept all night.

"No one slept," she said, holding hands and dancing with two girlfriends. "We all got on the phone and talked to each other. We all felt such joy."

Samuel Janvier, 21, said he left his house at 4 a.m. to join the celebrations at the palace. "I feel he can help us," Mr. Janvier said of Mr. Préval. "I can just feel it."

Up the mountain in the well-to-do suburb of Pétionville, a radio station owner, Anne Marie Issa, expressed the reservations of many of her peers. She wondered whether Mr. Préval would resolve all of his political challenges by summoning angry crowds out into the streets — a favorite tactic, she said, of Mr. Aristide. And she voiced the question that has seemed to dominate Haitian discourse: whether Mr. Préval would invite Mr. Aristide to return home.

"The way he has risen to power," Mrs. Issa said of Mr. Préval, "creates concerns among those of us who have lived a long history of intimidation, where leaders use the masses to impose their will."

Leslie Manigat, another former president and the runner-up in last week's election, was the most outspoken critic of the agreement that brought Mr. Préval back to power.

Official results from the first round of voting gave Mr. Manigat less than 12 percent of the votes, making clear that he stood little chance of defeating Mr. Préval in a second round. Still, Mr. Manigat said, he looked forward to that chance, and his supporters deserved it.

He described the agreement to declare Mr. Préval the winner as a "Machiavellian maneuver," and an "electoral coup," comparing it to the military takeover that ended his 1988 presidency only four months after he was elected.

Mr. Manigat said that he would not stand in the way of Mr. Préval's rise to power. But he added that Mr. Préval's presidency would be tarnished by a stain of illegitimacy.

"Violence has been rewarded," Mr. Manigat said during a news conference at his home Thursday morning. "As we did in the 1988 coup against us, we say good luck to the country."

Amy Bracken contributed reportingfor this article.