Tens of thousands of Shi'ite Muslims are pouring into Iraq's holy city of Kerbala, despite having become the main targets of a bombing campaign that followed the withdrawal of U.S. troops last month.

A political crisis since the U.S. withdrawal has raised fears among many Iraqis of a return to the sectarian slaughter in which tens of thousands were killed in 2006-07.

Attacks targeting Shi'ites have killed scores of people since the Shi'ite-led government of Prime Minister Nuri al-Maliki issued an arrest warrant for a Sunni vice president and the main Sunni backed party boycotted parliament.

Pilgrims have choked the streets of the capital Baghdad and border crossings with Iran this week for Arbain, one of the main holy days of the Shi'ite calendar.

Many pilgrims say they are fully aware of the dangers but are determined to commemorate the end of a 40-day mourning period for Imam Hussein, a grandson of the Prophet Mohammed who was killed in a 7th century battle in Kerbala.

I am not afraid to come to Iraq, but I have a 4-year-old child I left in Iran and I wrote my will before coming here, Zahraa Qaeli, a 32-year-old Iranian housewife from Khorramshahr, told Reuters at the Shalamcha crossing on the border near Basra.

Arbain has been a frequent target of militants since the U.S.-led invasion that toppled Sunni dictator Saddam Hussein, who banned Shi'ite festivals.

Scores of people have been killed and more than 200 wounded in attacks on pilgrims in the last five days. In the worst, a suicide attacker detonated a bomb at a checkpoint crowded with pilgrims near Nassiriya Thursday, killing 44, part of a wave of attacks that day in mainly Shi'ite areas that killed at least 73 people. A series of bombs in mainly Shi'ite districts of Baghdad on December 22 killed 72 people.

Nonetheless, traffic across the Iraqi capital has been diverted or tied up for days as thousands of defiant pilgrims make the 80 kilometre walk to Kerbala.

Tents are set up along main roads to serve food and drinks and provide beds for pilgrims, and ambulances stand at ready to offer emergency medical care.

'WELCOME MARTYRDOM'

I would definitely sacrifice my soul for Imam Hussein. Even if this political crisis is followed by bombings, that does not deter me from the pilgrimage along with my friends, said Abdul-Hussein al-Obeidi, 45, who carried a green flag bearing an image of Hussein.

We welcome martyrdom for the sake of Imam Hussein, said one of Obeidi's companions, who wore a blue track suit and walked with a cane.

Iraq in general, and Baghdad in particular, saw devastating sectarian violence between Shi'ite and Sunni Muslims in 2006-07, and is still plagued by a Sunni insurgency and Shi'ite militias.

I avoid some dangerous Baghdad districts, such as Doura, said Abdullah, 45, walking quickly with his brother as he pointed toward the former Sunni insurgent stronghold.

Many of the recent suicide and car bomb attacks against pilgrims bore the signature of methods used by the Iraq branch of al Qaeda.

I defy terrorists. They are a few gangsters, while we are the majority here, said Zahraa, 22, who wore a niqab that revealed only her eyes as she sat at a roadside tent in Baghdad. This is my fourth pilgrimage to Imam Hussein despite a trip of several days.

An official at the Shalamcha border point said 36,000 Iranians had crossed in eight days, and the number had climbed to 8,000 to 9,000 a day, with bigger numbers expected before the rite reaches a peak Friday and Saturday.

Iraqi officials usually put the number of Arbain attendees in the millions, many from mainly Shi'ite Iran.

This the first time I have come to Iraq. The situation here is great. We haven't encountered any problems, Iranian civil servant Muhsen Shehrazad, 27, said.

I am not afraid of the security situation. The love and passion we hold for Imam Hussein, peace be upon him, will not be dispelled by intimidation and explosions. Dying in the path of Hussein's passion is a wish.

(Editing by Jim Loney and Peter Graff)