At least 13 people were killed and scores were wounded in the Yemeni capital Wednesday when security forces and plainclothes government loyalists opened fire on more than 1,000 anti-government demonstrators, medical officials and witnesses said.

The clash, the most violent episode in Sanaa since at least 52 were killed March 18, erupted after security forces blocked a march by unarmed protesters demanding the departure of longtime President Ali Abdullah Saleh. Demonstrators then began erecting tents in front of rows of soldiers near a protest camp that Saleh’s opponents have occupied for weeks and, moments later, bullets rained down from snipers shooting from the upper floors of surrounding houses and from security forces on the ground.

A day earlier, protest organizers had announced an “escalation plan,” calling for an expansion of anti-government camps along major roads and highways across the country.

On Wednesday, as protesters ran from the hail of bullets, some were trampled in the stampede and tear gas filled the air. The dead and wounded were thrown over the backs of motorcycles or carried to the “Change Square” field hospital, where ill-equipped medical volunteers tended to them.

Abdul Malik al-Ansi, a doctor at the hospital, said late Wednesday that 13 had died and that more than 50 had been shot. Others on the scene said they expected the casualty figures to rise.

“They shot at us with .50-caliber machine guns, with AK-47s, with everything,” said Khalid al-Ansi, a leading human rights activist, as he helped the volunteers at the field hospital treat some of the wounded. “I don’t know how many are dead, but there must be scores.”

After the initial barrage, sporadic gunfire continued for hours around the one-square-mile protest camp.

“All I was holding was a chocolate bar and a cigarette when they opened fire on us,” said Anas Mansoor, 20, displaying a fistful of spent AK-47 shells he had picked up off the ground near the site of the shooting.

Anti-government demonstrators have taken up residence in makeshift tent cities across the country for more than three months.

The Tuesday statement from a group of protest organizers announced a 10-day escalation in demonstrations, involving general strikes, an expansion of protest camps, rallies and a march on the presidential palace next Tuesday.

Boone is a special correspondent.