A female suicide bomber mingling among Shia pilgrims in Baghdad detonated an explosives belt yesterday, killingatleast 54 people, officials said.

The bombing was the first major strike this year against pilgrims making their way to the southern city of Karbala to mark a Shia holy day. It came as a security official warned of a possible increase in attacks by insurgents using new tactics to bypass bomb-detection methods.

The bombing raises fears of an escalation of attacks as hundreds of thousands of Shias head to Karbala to mark onFridaytheendof40 days of mourning following the anniversary of the death Imam Hussein, a revered Shia figure.

The bomber hid the explosives underneath an abaya a black cloak worn from head to toe by women as she joined a group of pilgrims on the outskirts of Baghdad's Shia-dominated neighborhood of Shaab, said Maj. Gen. Qassim al-Moussawi, Baghdad's top military spokesman.

The bomber set off the blast as she lined up with other women to be searched by female security guards at a security checkpoint just inside a rest tent, al-Moussawi said.

A police official said 54 people, including 18 women and 12 children, were killed and 117 were wounded. A hospital official confirmed the casualties. Both officials spoke on condition of anonymity because they were not authorized to talk to the media.

Witnesses described a chaotic scene in the minutes after the blast.

Raheem Kadhom, 35, said he was standing nearby when a huge fireball erupted among the pilgrims.

Pilgrims were "on the ground, covered in blood and crying for help,” he said. "Banners were all over the ground and covered in blood.”

The blast was so powerful it knocked some out of their slippers and shoes, which were scattered across the ground, he said, adding how people put the wounded in cars, taking them to hospitals rather than wait for ambulances.

Despite an overall decline in violence in Iraq, al-Qaida and other Sunni extremists have routinely targeted pilgrims in an attempt to stoke sectarian strife and weaken the Shia-dominated government.

The vast numbers of pilgrims and the distances many of them must travel at predictable times of the year make it all but impossible to guarantee their safety against extremist groups. The pilgrims targeted Monday were walking from the northeast Diyala province and other areas north of Baghdad, police said.

During a pilgrimage in February of last year, a female suicide bomber attacked a tent filled with women and children resting during the walk to Karbala, killing 40 people and wounding 60 others. A month before that, a suicide bomber dressed in women's clothing and hiding among Iranian pilgrims killed more than three dozen people outside a mosque in Baghdad's Shia neighborhood of Kazimiyah.

Security forces were put on alert shortly after Monday's attack, al-Moussawisaid.