Baghdad, Iraq ? Thousands of Shiite youths, some armed, headed into Iraq’s capital Thursday for a pro-Hezbollah rally as skirmishes erupted with U.S. troops that left at least three people dead, officials said. Fifteen rallygoers were wounded in a bus bombing.

The city was rocked before the crowd arrivals by a motorcycle bomb that killed 12 people.

Two Marines were killed in Anbar province west of Baghdad, the U.S. military said.

Radical Shiite cleric Muqtada al-Sadr summoned his followers around the country to attend a mass rally today in the city’s Sadr City district in support of the Shiite militants of Hezbollah battling Israeli troops in southern Lebanon.

Iraqi government television said the Defense Ministry had approved the demonstration, a sign of the public anger over Israel’s offensive in Lebanon and of al-Sadr’s stature as a major player in Iraqi politics.

Crowds of young men, many wearing white shrouds, began arriving in eastern Baghdad’s Sadr City late Thursday and were housed in mosques and Shiite community centers. U.S. Army vehicles guarded approaches to the slum to prevent clashes between Shiite and Sunni extremists.

Some incidents were reported before the demonstrators reached Baghdad.

Iraqi police said one al-Sadr follower was killed by U.S. troops near Mahmoudiya after he brandished a weapon. American officials said two people were killed when gunmen in three vehicles shot at the guard towers of a U.S. base near the city and U.S. soldiers fired back.

At least 15 al-Sadr loyalists were injured when bombs exploded near a bus carrying them through southern Baghdad en route to Sadr City from southern Iraq, police Capt. Firas Geti said.

About 20 buses filled with al-Sadr followers drove to Baghdad from Basra, the country’s second-largest city. Most of the passengers were draped in the white shrouds that Muslims use to wrap their dead – a symbol of their willingness to die for Lebanon.