Story Highlights• Gunbattle with Mehdi militants happened during Iraqi-led raid on al-Sadr office
• Military arrests 72 suspected terrorists, 36 of them in Samarra after crackdown
• Samarra put on alert after insurgents give police three days to "repent" or die
• Death toll in Karbala blast climbs to at least 75, Interior Ministry official says
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BAGHDAD, Iraq (CNN) -- U.S. troops and fighters from radical Shiite Muslim cleric Muqtada al-Sadr's Mehdi Army clashed outside one of al-Sadr's main offices Sunday.
U.S. commanders said it was a raid led by Iraqi troops and aimed at capturing "high-value individuals."
Insurgents began shooting at U.S. troops as they took up positions around the meeting site in preparation for the raid, the U.S. command in Baghdad reported.
Iraqi state television reported of the incident that at least three civilians were killed and seven others wounded when "U.S. forces opened fire randomly on people" in the Bab Mrad area of Kadhimiya.
An Iraqi Interior Ministry official said the fighting began late in the afternoon, after the U.S. military arrived at al-Sadr's Kadhimiya office.
At least four Iraqis were taken to the Kadhimiya Hospital, the official said.
The U.S. military said several people arrested in Sunday's raid have been turned over to Iraqi security forces.
The growing threat of al-Sadr
Al-Sadr has long opposed the presence of U.S. troops in Iraq and led two uprisings against American and allied forces in 2004.
He now leads a key bloc in Iraq's parliament and recently ordered six ministers from his party to quit Prime Minister Nuri al-Maliki's government to press his demand that U.S. troops withdraw.
In December, a U.S. military report concluded the Mehdi Army has replaced al Qaeda as "the most dangerous accelerant" of the sectarian violence that has plagued Iraq for more than a year, and the buildup of U.S. troops in the Iraqi capital in recent months is aimed in part at curbing its influence. (Read full story)
Al Qaeda arrests
Separately, 11 unidentified bodies were found Sunday in Baghdad, an Interior Ministry official said.
Also on Sunday U.S. and Iraqi troops found 100 gallons of nitric acid and other bomb-making materials Sunday during raids that yielded the arrests of 72 suspected members of the al Qaeda in Iraq network, the U.S. military said.
The raids happened in Anbar and Salaheddin provinces, the military said. (Watch the top U.S. commander in Iraq explain his formula for success )
Half of those arrested were captured during raids in Samarra, in Salaheddin province north of Baghdad, according to the military. The nitric acid and bomb-making ingredients were found during a raid in Karmah.
The Samarra arrests came as authorities banned all movement in the city in response to recent threats, most notably a threatened suicide attack on the Samarra bridge, a security official said.
The Samarra bridge has a dam that generates power and regulates water levels on the Tigris River.
The city went on high alert after the Islamic State of Iraq -- the insurgent umbrella group that includes al Qaeda in Iraq -- distributed leaflets to police in Samarra on Saturday, warning them that they have three days to "repent" or be killed.
The insurgent group also told police to use loudspeakers at mosques and marketplaces to announce their rejection of the "apostate state."
A daylight curfew went into effect at 6 a.m. (10 p.m. Saturday ET) until further notice, the official said.
Two hours after the curfew took effect, a group of gunmen traveling in about 30 vehicles attacked a convoy of fuel tankers just outside Samarra, kidnapping the drivers of 16 trucks and setting fire to the vehicles, a Tikrit police official said.
The convoy was heading from Baiji -- the site of Iraq's largest oil refinery -- to Ramadi, in Iraq's volatile Anbar province west of Baghdad, the police official said.
In February 2006, Sunni extremists launched a strike against the Askariya Mosque, a Shiite shrine in Samarra.
The assault sparked reprisals against Sunnis by Shiite militias and death squads, and those strikes were countered by Sunni insurgents, including al Qaeda in Iraq, who attacked Shiite targets.
CNN's Jomana Karadsheh, Ingrid Formanek and Hugh Riminton contributed to this report.