Two Shiite militia leaders surrendered to American soldiers, while tens of thousands of supporters of hard-line Shiite cleric Muqtada al-Sadr streamed out of mosques to protest against an agreement that could keep US troops in Baghdad for years.
The arrests and demonstrations occurred on Friday, on the eve of Prime Minister Nouri al-Maliki's trip to Shiite-dominated Iran, his second visit there in a year.
US officials allege that Iran is arming and training Shiite militiamen and encouraging a public campaign in Iraq against the proposed US-Iraq security agreement, which the Iranians oppose.
One of those who surrendered yesterday allegedly ordered attacks on US troops, directed the kidnapping of Iraqis and helped smuggle Iranian weapons into Iraq, the US military said in a statement.
The other tried to escape by wading through an irrigation canal before turning himself over to US soldiers.
Names of the suspects were not released, but both were members of Iranian-backed "special groups," the US command said. The term is used by the American military to describe Shiite fighters who have defied al-Sadr's cease-fire order that ended seven weeks of fighting in Baghdad last month.
Iran denies arming the extremists, and it is unclear whether significant numbers of al-Sadr's Mahdi Army militia are really beyond his control.