Gunfire erupted when the protesters reached the square; it was sporadic at first, then more sustained. Security forces fired into the surrounding streets leading into the traffic roundabout.
Ambulances raced to the scene but had difficulty reaching those who had fallen in the barrage of bullets and tear gas, said marchers who made their way to the hospital with the injured.
"Everybody was on the ground. They were shooting at the heads and chest," said Mohamed Nabi, 27, who was with two friends in the emergency room. "We were chanting, 'Peaceful, peaceful!' but the government was willing to have a massacre. We weren't afraid. We were willing to die in this way."
Ali Hasan Arafat lay bare-chested on a stretcher, breathing heavily from the effects of tear gas. The gas was so caustic, he said, that he was blinded and ran into a signpost, injuring his head.
One doctor complained that the army had detained several ambulance drivers.
Rather than break its will, the crackdown appeared to energize the Shiite community. At the hospital, people cheered when a policeman said he was quitting the force to join them. Women and children headed to the facility intent on helping support the protests.
Opposition politicians sounded a defiant note before the violence Friday. Said Sheik Ali Salman, a leader of the main Shiite opposition Wefaq party: "It's the right of the people to demonstrate at any time."
Times staff writers Jeffrey Fleishman in Cairo, Paul Richter and David S. Cloud in Washington, Borzou Daraghi in New York and Carol J. Williams in Los Angeles contributed to this report.