A roadside bomb killed one Shiite pilgrim and injured three others after ripping through their car in a southern neighbourhood of Baghdad on Thursday, an interior ministry official said.
The pilgrims were attacked as they drove through the former Sunni insurgent stronghold of Dora en route to the holy Shiite city of Karbala 100 kilometres (60 miles) south of Baghdad, the official told AFP, requesting anonymity.
Roads heading towards Karbala have been crowded with Shiite pilgrims in cars and on foot ahead of ceremonies on Friday commemorating the birth of the Twelfth Imam, a disappeared ninth century leader of Shiite Muslims revered as a coming messiah.
Violence in Iraq has dropped off markedly in recent months, but attacks against security forces and civilians are common in the capital and the restive northern city of Mosul.
The number of violent deaths in Iraq fell by a third from the June figure of to 437 to 275 in July, the first month local forces have been in charge of security in urban areas since the US-led invasion.
The figure in May was 155, the lowest of any month since the 2003 US-led invasion.