BEIRUT, Lebanon — Lebanon’s feuding factions reached a breakthrough deal today that ends the country’s 18-month political stalemate, but also gives the militant Hezbollah group and its allies veto over any government decision.

The deal, reached with the help of Arab mediators, was immediately praised by Iran and Syria, which back Hezbollah. But it appears certain to accelerate fears in the West over Hezbollah’s new power.

Pro-government politician and parliament majority leader, Saad Hariri, seemed to acknowledge his side had largely caved in, spurred by a sharp outbreak of violence earlier this month after months of stalemate.

“I know that the wounds are deep and my injury is deep, but we only have each other to build Lebanon,” he said after the announcement of the deal, which was brokered after five days of talks in Qatar.

Hezbollah’s chief negotiator, Mohammed Raad, downplayed the group’s win.

“Neither side got all it demanded, but (the agreement) is a good balance between all parties’ demands,” he said.

The Bush administration seemed to be trying to put the best face on the deal even though it gave more power to Hezbollah, considered a terrorist group by Washington and Israel. An assistant secretary of State, David Welch, called the agreement “a necessary and positive step.”

The election of a compromise president — the head of Lebanon’s mostly neutral army — was expected Sunday, Lebanon’s state news agency reported.

The Hezbollah-led opposition won both its demands with the deal: veto power in a new national unity government, and an electoral law that divides up Lebanon into smaller-sized districts, allowing for better representation of the country’s various sects.

A few bursts of celebratory gunfire broke out in Beirut after the announcement. Lebanese television stations, which broadcast the Qatar ceremony live, showed Lebanese politicians and their Arab hosts congratulating and hugging one another.

The talks in Qatar and the deal were a dramatic cap to Lebanon’s worst internal fighting since the 1975-90 civil war. At least 67 people were killed when clashes broke between pro-government groups and the opposition in the streets Beirut and elsewhere earlier this month.

As Lebanon came close to a new all-out war, Arab League mediators intervened and got the sides to agree to hold last-ditch negotiations in the Qatari capital, Doha, to resolve the crisis.

But the resulting deal was a major victory for Hezbollah.

The opposition-allied Lebanese Parliament Speaker, Nabih Berri, also spoke at the Doha ceremony, saying that an opposition tent encampment across from the government building in downtown Beirut would be dismantled today.

Mr. Berri called such action a “gift” from the opposition, hailing the Doha agreement.

Within an hour, pickup trucks began hauling mattresses and supplies away from the encampment, which has paralyzed the commercial heart of the Lebanese capital for more than a year. Opposition supporters dismantled tents and took apart wooden boards used in the encampment.

In Iran, a foreign ministry spokesman, Mohammad Ali Hosseini, said the Lebanese deal was an “example of regional integration for achieving stability and tranquility.”

Syria also promptly endorsed the deal, with the Foreign Minister, Walid al-Moallem, saying “Lebanon’s security and stability are important and vital to Syria’s security and stability.”

The French Foreign Minister, Bernard Kouchner, said he was “personally very happy” about the Doha agreement and said it was now “up to all the Lebanese to use this accord to build the basis for national reconciliation.”

Prime Minister Fuad Saniora of Lebanon said the Lebanese should draw lessons from what happened and called on them to reject violence. He also called on Arab states to help support Lebanese forces, which kept a neutral role during the latest clashes.

“We must … pledge never to resort to arms to resolve our political differences,” Mr. Saniora said at the Doha ceremony. “We should accept each other and hold dialogue to solve the problems. We want to live together and we will continue that. We have no other choice.”

President Sarkozy of France said the deal was a “great success for Lebanon and all the Lebanese, whose courage and patience never failed despite the ordeals they have been through.”

As part of the deal reached at dawn today, Hezbollah and its political allies would receive veto power in the country’s new national unity government. The Syrian-backed opposition would get 11 seats in the Cabinet, while 16 seats would go to the American- and Western-backed parliament majority.

The remaining three would be distributed by the elected president. Previously, the opposition held six seats in the Cabinet.

The agreement, a copy of which was obtained by The Associated Press, states that the factions “pledged to refrain” from taking up weapons to resolve disputes and that the “use of arms or violence is forbidden to settle political differences under any circumstances.”

The government had sought a concession in Doha that Hezbollah would not again turn its guns on fellow Lebanese as in fighting earlier this month, but the broad clause referring to all Lebanese armed groups was apparently as much as it achieved.

The Telecommunications Minister, Marwan Hamadeh, said while the agreement “forbids internal use of weapons,” it also “calls for dialogue … on the whole subject of arms.”

Mr. Hamadeh also said both sides were satisfied with the new election law. The legislation is significant because it will determine how the sides distribute power in the capital and directly influence the outcome of the next parliamentary elections in 2009.

Lebanon has been without a president since Emile Lahoud stepped down in November, and rival factions have been unable to resolve their differences over a future government.

Both sides have agreed on the army chief, General Michel Suleiman, as a consensus candidate. But parliament had been unable to muster a quorum to meet because of disagreement on other remaining issues — including the formation of the national unity government and electoral law.

Mr. Hamadeh also said legislators from the parliament majority, who have been living abroad fearing for their safety after a wave of bombings targeting mainly anti-Syrian lawmakers and politicians, would be asked to return to Beirut to vote for the president in parliament.

The agreement was struck after host Qatar stepped up pressure yesterday, offering the rival factions two drafts on how to end the deadlock and a day to consider the proposals.

The 18-month standoff started when Hezbollah-led opposition lawmakers resigned from the government in November 2006 to protest the Cabinet’s refusal to grant them enough seats to ensure veto power.

The Qatar deal was also a triumph for the tiny energy-rich Gulf state. The Lebanese stalemate had defied mediation efforts by other Arab and European countries, including shuttle diplomacy in the last year by the foreign minister of France, Lebanon’s former colonial ruler.