In the most violent demonstration, about 100 people threw stones and a firebomb at the British Embassy in Tehran, damaging the building but harming nobody as they accused Britain and the United States of being accomplices in Israel's fight against Hezbollah, a Shiite group in Lebanon that is backed by Persian Iran.
Even Sunni Muslim demonstrators took to the streets of Damascus, Cairo and Amman. But their numbers were dwarfed by the huge Shiite turnout in Baghdad, organized by anti-American cleric Muqtada al-Sadr.
Crowds of al-Sadr supporters from across Iraq's Shiite heartland converged on the capital's Sadr City district, chanting "Death to Israel, Death to America" in the biggest pro-Hezbollah rally since the conflict began July 12.
Demonstrators wearing white shrouds symbolizing willingness to die for Hezbollah waved the guerrillas' banner and chanted slogans in support of their leader, Sheik Hassan Nasrallah.
"Allah, Allah, give victory to Hassan Nasrallah," the crowd chanted before burning Israeli and American flags.
Al-Sadr and his supporters could spell real trouble for American soldiers on the ground, reports CBS News correspondent Mark Strassmann.
"They would like to get the Americans out," former Ambassador Peter Galbraith, a critic of the Iraq War, told Strassmann. "The attacks on Lebanon become a rallying cry, a vehicle for which they could inflame popular sentiment against the United States."
Organizers and local police said hundreds of thousands attended the rally, but the U.S. military later estimated the crowd at 14,000. Associated Press reporters at the scene thought attendance was at least in the tens of thousands during the high point of the march.
In other developments:
The rally in Baghdad went off peacefully — a remarkable achievement in a city where bombings and shootings are an everyday occurrence. Sadr City is under the effective control of the cleric's Mahdi Army militia, which maintains its own security network.
However, five busloads of Shiite demonstrators were ambushed southwest of Baghdad late Friday as they returned home from the rally, police said. Imam Ali hospital in Sadr city received 14 wounded from the attack, who told them that three others had been killed, Rasool Qasim al Zibon director of media office in hospital said.