A suicide bomber driving a car has killed at least 27 Shia Muslims at a bus station in the Iraqi town of Mussayab.
The attack, which also wounded at least 60, happened as people were gathering to return home from a religious rite.
Police said the bomber drove into a busy bus station where pilgrims were catching buses back to Baghdad and the northern provinces after the Arbain rite in the holy city of Kerbala, where thousands make an annual pilgrimage.
Mussayab is 60km south of the capital Baghdad.
Arbain has been a frequent target for militants since the US-led invasion that toppled Sunni dictator Saddam Hussein, who banned Shia festivals.
A roadside bomb targeting a minibus transporting Shia pilgrims back from Kerbala also wounded eight people in New Baghdad.
The latest violence followed nearly two weeks of protests against Shia Prime Minister Nuri al-Maliki by thousands of people from the minority Sunni community in the western province of Anbar, which shares a border with Syria.
The protesters accuse Mr Maliki of being under the sway of non-Arab Shia neighbour Iran and of marginalising Sunnis, who dominated Iraq until the 2003 invasion.
They want Mr Maliki to abolish anti-terrorism laws they say are used to persecute them.
No group claimed responsibility for Thursday's attacks, but Iraq is home to several Sunni insurgent groups including a local branch of al-Qaeda, the Islamic State of Iraq, which often targets Shia, seeking to re-ignite sectarian strife.