Police opened fire when protesters tried to break through a police cordon. They also sprayed them with tear gas as tens of thousands spilled out of Change Square, where many youths have camped out since early this year demanding President Ali Abdullah Saleh end his 33-year rule.
Injured protesters were rushed to a field hospital in the square on stretchers. "This is the worst day I've seen in three months. We're expecting more dead to come in," said Dr. Jamal Al-Hamdani, who was treating patients.
A Reuters witness saw dozens of men slumped on the ground, overcome by tear gas inhalation. Men on motorbikes and ambulances whisked them away from the scene.
Saleh, recovering in Saudi Arabia from a June assassination attempt, is holding onto power despite international pressure to quit and eight months of protests, which have paralyzed the country.
The Defense Ministry said on its website protesters threw petrol bombs, setting a police car ablaze. State media blamed gunmen belonging to opposition parties for opening fire on the march.
Frustrated by Saleh's tenacity and their failure to dislodge him, protesters are seeking to ratchet up demonstrations, which have dragged into their eighth month.
"Escalation, escalation," they chanted, flooding side streets where there were large numbers of security forces and armed anti-Saleh tribesmen, who have thrown their weight behind the popular uprising.
"The pain and the bullet won't be a problem," said one protester with a heavily bandaged foot. "What we need is to get rid of this regime ... I could go on until they kill us all," Radwan Qasem, 37, told Reuters.
Earlier on Sunday, fighting broke out in a northern district of Sanaa, the latest breach of a cease-fire between tribesmen opposed to Saleh and troops loyal to him.
Shelling could be heard near the home of a prominent anti-Saleh tribal leader in the Hasaba district, the site of weeks of fighting. The family of Sadeq Al-Ahmar said the Republican Guard, commanded by Saleh's son, had shelled their house.
The protest movement has stepped up demonstrations in the past week, angered after Saleh authorized his vice president to negotiate a power-transfer deal. Many believe the move is just the latest of many delaying tactics. At the same time, greater numbers of security forces and armed regime supporters have also been turning out in the streets in recent days.
