BAGHDAD, Iraq — Facing criticism from the United Nations and threats of a boycott from the Sunni minority, the Iraqi National Assembly yesterday reversed a controversial rule change that would have made it more difficult for opponents of the country's draft constitution to vote it down in a nationwide referendum to be held in 10 days.

The modification came several hours before a bomb exploded at the entrance of a Shiite mosque south of the capital, killing 25 people and injuring dozens more on the first day of the Shiite observance of the holy month of Ramadan.

The attempt by Shiite and Kurdish assembly members to change the voting rules had infuriated Sunni political leaders.

Iraqi law states that the proposed constitution will be rejected if "two-thirds of the voters" in at least three of the country's 18 provinces vote against it in the Oct. 15 referendum. The parliament voted Sunday to interpret that to mean two-thirds of a province's registered voters, rather than two-thirds of those who cast ballots, drawing objections from the United Nations and Sunni Arab politicians who threatened a boycott because they said the ruling virtually guaranteed the document would pass.

But under international pressure and with some Sunni leaders threatening to boycott the referendum, the Shiite- and Kurdish-dominated parliament changed course. Yesterday, the National Assembly voted overwhelmingly to define "voters" as those who show up at the polls, meaning fewer "no" votes would be required to defeat the constitution.

Sunni Arabs, who make up about 20 percent of Iraq's population but hold majorities in some provinces, oppose the constitution because they believe it is too favorable to Shiites and Kurds. They also fear the document will lead to a weakened central government and subsequent splintering of the nation.

Shiites and Kurds controlled the drafting process, and their supporters, who make up the majority of the country, are expected to vote overwhelmingly in favor of it.

U.S. and Iraqi officials have long expressed hope that the process of drafting and passing a permanent constitution to replace an interim document written a year ago would help unite Iraq's religious and ethnic factions under a national banner.

If the Sunnis had gone through with a boycott, it could have jeopardized the legitimacy of the vote in the eyes of Sunni Arabs as well as the rest of the Arab world.

The bombing in Hillah marked a bloody beginning for Ramadan, which commemorates the revelation of the Quran to the prophet Muhammad. Shiite worshipers who had fasted all day were gathering for the traditional evening meal known as iftar when the bomber struck.

The attackers planted the explosives at the entrance to the Ibn al-Nama mosque in Hillah, about 60 miles south of Baghdad. Insurgents have repeatedly launched attacks in Hillah throughout the 2 ½-year old insurgency. Last week, a suicide attacker killed 10 people when he detonated his car bomb in a crowded market. In February, a suicide car bomber targeting police and army recruits in the city killed 125, the single deadliest attack of the war.

U.S. and Iraqi officials have predicted a spike of violence for Ramadan and the days before the Oct. 15 referendum.