Thousands of Shiite pilgrims were determined to converge on their holy shrine in Baghdad's northern district of Kadhimiya on Tuesday for a religious ceremony, where thousands of extra police and soldiers were deployed, a traffic curfew was imposed and additional checkpoints were set up to avert another suicide bombing.

On Monday three suicide bombers, believed to be women, detonated their explosive belts in Baghdad in the midst of a crowd of pilgrims heading to the shrine, killing more than 25 people and injuring another 75.

The mass pilgrimage, in which 1 million people are expected to take part, is to mourn the death of Shiite imam Moussa Kadhim 12 centuries ago, believed to have been buried in the Kadhimiya mosque after allegedly being poisoned in Baghdad by agents of the ruling Sunni caliph, Harun Rashid.

Iraqi critics say that although security sources had information the pilgrimage would be targeted again this year, having been a previous target for attacks since the Shiites began marking the occasion following the U.S.-led toppling of President Saddam Hussein's secular regime in 2003, the suicide bombers still managed to infiltrate the crowds on Monday.

The suicide attack and shootings in the northern oil-rich city of Kirkuk on Monday, in which 27 people were killed and at least 150 others were wounded in a demonstration, also did not prevent thousands of people from marching in the Kurdish city of Erbil the following day against a controversial elections bill.

Local security forces in the autonomous region were heavily deployed along the 10-kilometer march to the Kurdish parliament, where demonstrators demanded that the elections law be revoked and that Kirkuk become part of the Kurdish region.

In what has been described as the worst act of violence in Kirkuk, which threatens a serious security collapse in the city that has been relatively calm since 2003, a bomber blew up, causing a stampede and guards opening fire.

Kurdish officials said the victims were those who were running away after the explosion and were caught in a wave of random shootings by guards and demonstrators.

Unlike in Kirkuk, the demonstration in Erbil ended peacefully, as Kurds denounced the electoral bill, which was endorsed by the 275-member National Assembly in Baghdad this month, as "unconstitutional."

The Kurdish bloc had boycotted the voting session and the bill was later vetoed by President Jalal Talabani, a Kurd.

The controversy over the law is expected to delay the local elections, which analysts said would be a battlefield in the power struggle between the sectarian parties that threatens to redraw and divide the country's political map.

At issue for the Kurds regarding the law is an article that fails to mention a referendum on Kirkuk's fate, which was to be held by the end of 2007 but delayed at the recommendation of the United Nations.

The Kurds, which the Arabs complained have been expelling Arabs and enticing Kurds to settle in Kirkuk to claim the oil wealth and annex the city to the Kurdistan region, are also protesting the bill's fixed and equal allocations to individual candidates from the largest three groups: the Kurds, Arabs and Turkmen, while the minority Christians get two seats each.

Iraqi analysts warn that the suicide bombings in the city on Monday indicated that the political-ethnic tensions over the future status of Kirkuk could turn violent, or at least could provide fertile cause for violent elements, such as al-Qaida, to operate more easily if the differences were not settled.

Security sources say that Monday's deadly attacks in Kirkuk and Baghdad had al-Qaida's fingerprints, but could not confirm the group's responsibility.

Nevertheless, Iraqi and U.S. forces on Tuesday launched a major offensive against insurgents and al-Qaida gunmen in Diyala province in northeastern Iraq, which the Iraq-U.S. alliance hopes will be the network's last stronghold in the country, after having flushed out suspected elements from other provinces in similar sweeps.

Iraqi officials said that 30,000 Iraqi troops were taking part in the operation, during which soldiers raided neighborhoods in Baquba and arrested an unknown number of suspected gunmen so far.

The U.S. army in Iraq says the offensive was planned and executed by Iraqi forces, while the American forces were only providing logistic support.

"The goal of the operation is to seek out and destroy criminal elements and terrorist threats in Diyala and eliminate smuggling corridors in the surrounding areas," AFP quoted U.S. Army spokesman Maj. John Hall as saying. "We look forward to reducing our support footprint as security conditions on the ground permit."

Iraqi military officials have described Diyala as the "most dangerous province in Iraq," where Sunni and Shiite insurgents are said to be operating freely, having virtually kept the central authorities out of the area.

Analysts say the offensive in Diyala, an important strategic province linking central and northern Iraq and borders Iran, might be more difficult to subdue than other areas, and will not likely be the last if the insurgents hold on to their weapons and flee elsewhere.