US forces opened fire from helicopters during an overnight clash with Shi'ite militants in western Baghdad, killing 18 people, the US military said.
Angry mourners, chanting and raising fists, took to the streets in the Shula district of the Iraqi capital, carrying wooden coffins of those killed in the clash.
An Iraqi police source at nearby al-Hakim hospital said 13 bodies had been brought to its morgue, including one woman. Another 13 wounded were treated there.
After years in which Sunni Arab insurgents were their main enemies, US forces now say nearly three quarters of attacks on them come from the country's newly empowered Shi'ite majority.
The clash came while women and children were sleeping on rooftops to keep cool. Reuters Television pictures showed cars with their windows blown out after being strafed by gunfire.
"We demand the Iraqi government and parliament stop the Americans interfering in Shula," local tribal elder Sabeeh al-Sharji said. "As you can see, civilians sleep on the roofs. These random attacks terrify women and children."
The US military said a group of its paratroops had come under small arms and machine gun fire while searching for a weapons cache, and "confirmed killing 18 enemy combatants".
The political movement of anti-American Shi'ite cleric Moqtada al-Sadr said four women were killed.