BAGHDAD — Shiite militiamen took to the streets of Baghdad on Saturday in a massive show of force, marching by the thousands with a vast array of arms.

The fighters were loyal to one of Iraq's most divisive figures, Muqtada al-Sadr, who was among America's deadliest enemies during the Iraq war. Sadr and his Mahdi Army plagued U.S. forces in the bloodiest years of the occupation; the hardline cleric is despised by many of Iraq's Sunnis as well. He had frozen his army's militant activities in recent years — and the sudden resurgence of his well-armed fighters in the capital threatened to bring sectarian tensions to a dangerous new height.

The marchers called themselves the "Peace Brigade" — a rebranding meant to distance the group from its dark past — but the demonstration was a clear-cut threat of war against the Sunni militants who have overrun much of northern Iraq.