BAGHDAD, Iraq – Under American and U.N. pressure, Iraq’s Shiite-led parliament yesterday reversed its last minute electoral law changes, which would have ensured passage of a new constitution but which the United Nations 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 American 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 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 October 15 referendum. Al Qaeda 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 American 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 American 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. America in particular is eager to see the passage of the charter, seen as key to beginning the withdrawal of some American forces.