BAGHDAD (AP) — Tens of thousands of Iraqis massed in Baghdad's Tahrir Square on Friday in the biggest demonstrations since anti-government protests erupted a month ago, defying security forces that have killed scores of people and harshly criticizing Iran's involvement in the country's affairs.

The square and the wide boulevards leading into it were packed with flag-waving protesters, as security forces reinforced barricades on two bridges leading to the heavily-fortified Green Zone, the seat of government. The protesters want sweeping change to the political system established after the 2003...