Syrian security forces fired bullets and tear gas Friday at tens of thousands of protesters across the country, killing at least 75 people in the bloodiest day of the monthlong uprising and signaling that the authoritarian regime was prepared to turn more ruthless to put down the revolt against President Bashar Assad.

Among the dead were a 70-year-old man and two boys ages 7 and 10, Amnesty International said. In the southern town of Izraa, a man ran carrying the body of a young boy, whose hair was matted with blood from a gaping wound on his head, as another child wept and shouted.

In other towns, protesters scattered for cover from sniper bullets, then dragged corpses through the streets. Mobile phone images showed the bodies lined up on the floor inside buildings.

The rallies, most marching out from mosques after Friday's noon Muslim prayers, erupted in towns and cities stretching along the breadth of the country, including in at least two suburbs of the capital, Damascus.

The death toll was likely to rise, raising fears that there will be an explosion of violence Saturday as relatives bury their dead in funerals that in the past have turned into new protests. Ammar Qurabi, head of Syria's National Organization for Human Rights, said another 20 people were missing.

Friday's toll was double that of the previous deadliest day of the uprising, on April 8, when 37 were killed around the country. The heavier crackdown came after Assad warned a week ago that any further unrest would be considered "sabotage" after he made the gesture of lifting long-hated emergency laws, a step he ratified on Thursday.