In the worst incident at least 20 Iraqi protesters and two soldiers - one American, one Salvadoran - were killed and more than 200 people injured in Kufa, near Najaf, during a demonstration to denounce the closure of a militant Baghdad newspaper and the arrest of an aide to the hardline Shia cleric Moqtada al-Sadr.
The protests spread to cities across the south and to Baghdad, where seven US soldiers were killed and at least 24 injured trying to regain control of the eastern Shia slums. British troops clashed with Shia protesters in the southeastern town of Amara, and a number of Iraqis were injured in gunbattles.
Earlier, two US marines were killed in the restive Sunni triangle west of Baghdad, bringing to 600 the number of US troops killed since the start of the war.
There were further bombings meanwhile in Kirkuk and Samarra and a Shia mosque north of Baghdad, while an oil pipeline was set alight in southern Iraq.
The security crisis prompted a leading US Republican, Senator Richard Lugar, to question whether the Bush administration's June 30 deadline for turning over sovereignty to the Iraqis should be extended.
Troops clash with Shias, page 4 Secular success in polls, page 16