Under U.S. and U.N. pressure, Iraq's Shiite-led parliament Wednesday reversed its last-minute electoral law changes, which would have ensured passage of a new constitution but which the United Nations (search) called unfair.

Sunni Arab leaders who had threatened a boycott because of the changes said they were satisfied with the reversal and were now mobilizing to defeat the charter at the polls. But some warned they could still call a boycott to protest major U.S. offensives launched over the past week in western Iraq, the Sunni heartland.

Also, a bomb exploded at the entrance of a Shiite mosque south of Baghdad, killing at least 25 and wounding 87, as hundreds of worshippers gathered there for prayers at the start of the Islamic month of Ramadan (search) and for the funeral of a man killed two days ago in a bomb blast at his restaurant.

It was the latest in a string of insurgent attacks — targeting Shiite Muslims in particular — aimed at wrecking the Oct. 15 referendum. Al Qaeda (search) in Iraq, which has declared "all-out war" on Shiites, has called for stepped up violence during Ramadan. More than 270 people have been killed in the past 10 days.

Thousands of U.S. troops were waging two major offensives in western Iraq, the Sunni heartland, in an attempt to put down insurgents ahead of the vote.

The reversal of the election changes passed by parliament over the weekend was a political victory for U.N. and U.S. officials, boosting chances that Sunnis will see the referendum as fair and participate, thus giving the outcome credibility.

Yet that success restored the possibility that Sunnis will manage to veto the constitution, which would prolong Iraq's political instability. The United States in particular is eager to see the passage of the charter, seen as key to beginning the withdrawal of some U.S. forces.

Saleh al-Mutlaq, a top Sunni politician, said the boycott threat over the election law was lifted. "With this result, the Sunni Arabs will be able to defeat the constitution," he told The Associated Press. "I am sure if there is honesty (in the election process) 95 percent of the Sunni Arabs will vote 'no."'

But he warned that Sunnis might still call a boycott if the U.S. offensives do not end soon, saying the turmoil will suppress Sunni voting.

"If the ongoing military operations continue, we will have no other choice but to call for a boycott simply because we will be having no role to play in the political process to tell our opinion about the constitution," he said.

U.S. commanders have said operations Iron Fist (search) and River Gate (search), in towns along the Euphrates River, will be finished ahead of the referendum. The twin offensives aim to break insurgents' hold their so residents feel safe going to the polls.

Meanwhile, members of the ruling Shiite-led coalition lay the groundwork to challenge the results if Sunnis succeed, expressing fears insurgent violence could prevent pro-constitution voters from going to the polls, swinging key Sunni provinces toward rejection.

While parliament reversed voting rule changes that would have made it nearly impossible for Sunnis to defeat the constitution, it also underlined that it would appeal voting results if it believed insurgent attacks or threats had affected the balloting.

"The National Assembly and the judicial system should take people's complaints seriously if they say they cannot vote freely because of terrorism," said deputy speaker Hussain al-Shahristani.

In its decision Wednesday, parliament underlined that security forces must remain in place even after Oct. 15 to deter or prevent reprisal attacks against voters. It also said poll employees must be vetted to ensure they don't spy for the insurgents by handing over the names of voters.

The United States said it was better to have fair rules to ensure strong Sunni turnout. "The constitution has been written. Folks will have a chance to vote it up or down," President Bush said.

The bomb blasted the entrance of the Husseiniyat Ibn al-Nama mosque in the southern town of Hillah at 6 p.m., ripping through strings of lightbulbs and green and red flags hung to celebrate the Muslim holy month of fasting, which began Wednesday for Shiites and Tuesday for Sunnis.

"We heard an explosion and then I fainted. I woke up when policemen splashed water over my face, and I saw all the damage, the martyrs and the wounded," said Haj Mohammed Abdullah, a 45-year-old shopkeeper.

"How could they do that?" he cried, referring to the attackers.

The blast, which police believed was caused by a planted explosive, killed at least 25 people and wounded at least 87, said Dr. Adnan al-Nashtah (search) of the city's health department.

It was the second major bomb attack in a week in Hillah, one of the most insurgent-hit towns in southern Iraq, about 60 miles from Baghdad.

Iraq's Shiite majority, concentrated in the south, and the Kurdish minority in the north overwhelmingly support the constitution. Sunni Arabs make up only 20 percent of the population but can still defeat the constitution if they get a two-thirds "no" vote in any three of Iraq's 18 provinces. There are four provinces where they have a chance of doing so.

The Shiite-led parliament tried to close that loophole Sunday by issuing a new interpretation of the rules, saying two-thirds of registered voters had to vote "no" — not two-thirds of those casting ballots. That raised the bar for rejection, and outraged Sunnis threatened a boycott.

The United Nations called the change unfair, and U.N. and U.S. officials pressed the government to reinstate the original rules.

On Wednesday parliament did so. The resolution passed 119 to 28, but some Shiite members staunchly defended the initial change.

"This will mean that the opinion of the majority will be crushed by the opinion of the minority," said lawmaker Maraim al-Reis, denouncing the U.S. and U.N. pressure. "We could have 30 people vote in three provinces, with 20 voting no — so 20 persons can bring down a constitution approved by 6 or 7 million."

U.N. Secretary-General Kofi Annan (search) welcomed the reversal but expressed disappointment that the referendum process had not united Iraqis.

"We support an all-inclusive process and we had hoped that this electoral process ... would pull the Iraqis together," Annan said. "It has not worked as we had hoped, but we still urge the parties to work together and I believe the reversal by the parliament of the decision last night would help the process."