(Updates with end of rite, bomb blasts)

By Sami al-Jumaili and Wisam Mohammed

KERBALA, Iraq, Feb 16 (Reuters) - Roadside bombs killed eight Iraqis in Baghdad on Monday as crowds of Shi'ite Muslim pilgrims began an arduous trek home at the end of a major rite, some catching rides on army trucks because of a lack of buses.

Defying the suicide bombs that have threatened gatherings of Shi'ite Muslims since the 2003 U.S.-led invasion toppled the Sunni-led government of Saddam Hussein, hundreds of thousands if not millions poured into the holy city of Kerbala for Arbain.

Beating heads and chests in ritual mourning for Hussein, the Prophet Mohammad's grandson, who died in a seventh century battle, pilgrims streamed through the Imam Hussein mosque in the city 80 km (50 miles) south of Baghdad in an endless procession.

The rite culminated at Hussein's gilded grave, where worshippers wept and prayed, while giant television screens in the city showed films of Hussein's deeds and preachers chanted and recounted Shi'ite tales through loudspeakers.

The government called on people with cars to give fellow pilgrims lifts home, and scores scrambled onto army trucks brought into town because of a shortage of buses.

"I walked for many days to reach Kerbala. Now we just want to go home but we can't find a bus," complained Omran Kadhom, 45, as he stood in a bus terminal in Kerbala.

The deployment of 30,000 Iraqi soldiers and police failed to prevent a bomb attack in Kerbala on Thursday that killed eight people near the revered shrine, and a suicide bombing on Friday on a route to Kerbala that killed 42.

Saturday and Sunday, however, passed without bloodshed in Kerbala, the peace broken only on Monday when two roadside bombs targeted Shi'ite areas in Baghdad as pilgrims returned.

The first bomb killed four and wounded 13 on a minibus in the sprawling slum of Sadr City, a stronghold of supporters of anti-American Shi'ite Muslim cleric Moqtada al-Sadr. The second also killed four and wounded 13 in a minibus, this time in Kamaliya, another Shi'ite area.

Officials in Kerbala say that around 10 million Shi'ite pilgrims flooded the city this year for the annual ritual, once suppressed like other Shi'ite gatherings under Saddam and which marks the end of 40 days of mourning for Hussein.

Violence in Iraq has fallen significantly in recent months but suicide and car bomb attacks remain common.

U.S. officials say the attacks are attempts to reignite the sectarian slaughter that almost tore Iraq apart.

Politics may also be a factor after allies of Shi'ite Prime Minister Nuri al-Maliki posted gains in Jan. 31 local elections.

That set Maliki's Dawa Party up for a muscular run in parliamentary polls at the end of the year, and rivals may want to undermine the perception that he can take some credit for the reduction in bloodshed, security sources say. (Writing by Michael Christie; Editing by Giles Elgood)

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