Four Bulgarian and two Thai soldiers, as well as seven Iraqis, were killed instantly. At least 30 occupation troops in the Polish-led force and more than 130 Iraqis were also reportedly wounded in the coordinated attacks in which four car bombs exploded.
Mortars and machine-gun fire were also used.
"Four attacks were carried out in Karbala, two against camps of our multinational coalition and one against the building of the administration of Karbala," said Polish General Andrzej Tyszkiewicz in Iraq.
"The attacks were made with cars packed with dynamite, mortars and automatic weapons fire," Tyszkiewicz said. The largest attacks yet in the Polish-led zone, they were "well planned, coordinated, massive and simultaneous".
Seven Iraqis were reported killed in the attacks and 135 wounded. Four of the Iraqi dead were members of the Facilities Protection Services, an auxiliary security force set up by Iraq's US-led occupation authority to defend key infrastructure from attacks by the eight-month-old resistance.
Occupation forces deployed rapid reaction forces to counter-attack, including 10 helicopters, Tyszkiewicz said. All of the division's more than 9000 soldiers have been placed on alert and a curfew could be imposed in Karbala, a spokesman for the Polish military said in Warsaw.
At least six foreign soldiers have
been killed in the mostly Shia city
Poland has lost one soldier in combat in Iraq since the start of the US-led war in late March, while at least 210 US soldiers have been killed in action since US President George Bush declared major hostilities over on 1 May.
The violence has also claimed the lives of British, Italian, and Spanish military personnel, as well as diplomats or contractors from Colombia, Japan, and South Korea.
US soldiers wounded
Also on Saturday morning, five US soldiers were wounded when their patrol hit two improvised explosive devices (IED) in the Rusafa district of Baghdad east of the Tigris River, a US military spokesman said.
The first IED struck the patrol at 7:25am (0425 GMT). Soon after, as "a quick reaction force was on the way" to the site of the incident, it too was hit with another explosion, according to the spokesman from the 1st Armoured Division, which commands the capital.
The spokesman said he had no further details on the incident.