Three female suicide bombers killed 28 people and wounded 92 when they blew themselves up among Shi'ites walking through the streets of Baghdad on a religious pilgrimage yesterday, Iraqi police said.

In the northern oil city of Kirkuk a suicide bomber killed 22 people and wounded 150 at a protest against a disputed local elections law, Iraqi health and security officials said.

One security official said the bomber may also have been a woman.

The attacks mark one of the bloodiest days in Iraq in months and underscored the fragility of recent security gains in the country, where violence is at its lowest level since early 2004.

There was no immediate claim of responsibility for the Baghdad blasts but Sunni Islamist al Qaeda often targets Shi'ite pilgrims. It considers Shi'ism - the majority Muslim denomination in Iraq - heretical.

At least one million people are expected to take part in the pilgrimage in the Iraqi capital, which peaks today and marks the death of one of Shi'ite Islam's 12 imams, one of the most important events in the Shi'ite religious calendar.

"These blasts that happened today will increase our determination to finalise this ceremony... and defeat terrorism," pilgrim Taher Abd-Noor said.

Al Qaeda has increasingly used women to carry out suicide attacks because they can often evade the more stringent security checks applied to men. Women have carried out some 20 suicide attacks in Iraq this year, the US military has said.

The apparently coordinated blasts in Baghdad shattered a period of relative calm in the city and took place despite heavy security for the annual pilgrimage to the Kadhamiya shrine. US commanders caution that despite better security, suicide bombers wearing vests packed with explosives will still periodically manage to slip into crowded places.

In Kirkuk, Kurdish television footage showed thousands of people demonstrating against Iraq's provincial elections law when an explosion prompted a rush for cover. A Reuters witness said there was a stampede as police fired into the air.

A security official who declined to be named said witnesses saw a female suicide bomber carry out the attack and that the body parts of a woman had been found, distinct from other victims of the blast because it was completely dismembered.

Tensions have been high in oil-rich Kirkuk before provincial polls expected to be held late this year or early 2009.