SANAA, Yemen — As snipers hidden on rooftops fired methodically on Yemeni protesters today, police sealed off a key escape route with a wall of burning tires, turning the largest of a month of anti-government demonstrations into a killing field in which at least 40 people perished.

Many of the victims, who included children, were shot in the head and neck, their bodies left sprawled on the ground or carried off by other protesters desperately pressing scarves to wounds to try to stop the bleeding.

The dramatic escalation in President Ali Abdullah Saleh’s crackdown suggested he was growing more fearful that the unprecedented street protests set off by the region’s unrest could unravel his 32-year grip on power in the volatile and impoverished nation.

The United States, which supports Saleh’s government with hundreds of millions of dollars in military aid to battle one of al-Qaida’s most active franchises, condemned the violence. President Barack Obama said those responsible must be held accountable, and Saleh should honor his pledge to allow peaceful demonstrations.

Instead, Saleh declared a nationwide state of emergency that formally gave his security forces a free hand to confront demonstrators. There was no word on how long the emergency laws would remain in force.