A suicide bomber has killed nine people and wounded 15 in an attack on a Shia mosque during Friday prayers, Iraqi police say.
Reports say the bombing took place in the town of Musayib, 80km (50 miles) south of the capital, Baghdad.
The attack comes a day after the Iraqi parliament backed a deal regulating the future presence of US troops.
Iraqi Shia cleric Moqtada Sadr has declared three days of mourning in response to the parliament's decision.
Under the agreement, US troops will leave Iraqi streets by mid-2009 and quit Iraq entirely by the end of 2011.
Levels of violence in Iraq have fallen to a four-year low, but bombings continue on an almost daily basis.
Moqtada Sadr has the support of 30 MPs in the 275 member parliament and has thousands of gunmen under his command in his militia, the Mehdi Army.
We offer our condolences to the Iraqi people over this calamity that has fallen upon them with the signing of the agreement of humiliation and indignity
Statement by Moqtada Sadr
The militia is at present observing a ceasefire with the Iraqi government and US forces.
He has constantly opposed the US military presence and wants Iraq to expel American troops and end all negotiations with Washington.
In a statement he told his supporters to put up black flags, organise mourning ceremonies and hold peaceful demonstrations. Some began in Sadrist controlled areas after Friday prayers.
Whilst Sadrists and their militia do control some areas of the country analysts believe their sway over the Shia community is reducing, BBC Baghdad correspondent Humphrey Hawksley reports.
It is widely thought Moqtada Sadr himself spends much of his time in Iran and that the major influence still lies with the Grand Ayatollah Ali Sistani.
He works in the holy city of Najaf and routinely holds talks with both Iraqi and foreign officials.
Ayatollah Sistani's influence was seen as key to the way the agreement with the US was approved, our correspondent says.
He insisted that the minority Sunni politicians were on board so it could be seen to encompass the interests of as many Iraqis as possible.