SANAA, Yemen -- Security forces opened fire Sunday on tens of thousands of demonstrators in Yemen's capital, Sanaa, killing at least 20 protesters in one of the bloodiest days of the 9-month-old rebellion against President Ali Abdullah Saleh.
Violence broke out when protesters marched from Sanaa University toward heavily guarded government buildings. Most demonstrators were peaceful, although some hurled stones and gasoline bombs after snipers shot from rooftops and troops loyal to Saleh opened fire with high-caliber weapons.
"We were walking and chanting 'Peaceful, peaceful,' " said Hamdi Mohammed, a demonstrator. "But then the soldiers attacked us and we threw rocks and gasoline bombs. They opened fire with machine guns and rocket-propelled grenades. It was horrible what they did to us."
Anti-government protesters have grown increasingly frustrated over Mr. Saleh's refusal to resign. The president is recuperating in Saudi Arabia from injuries suffered in a June rocket attack on his residential compound. Mr. Saleh has repeatedly broken promises to step aside, and his latest proposal to transfer power to his vice president was viewed by protesters as yet another trick.
The nation is slipping precariously close to civil war, with Mr. Saleh's government and military under increasing pressure from well-armed tribes, hundreds of defected soldiers led by Gen. Ali Mohsen Ahmar and an al-Qaida affiliate that has been battling for control of villages and cities.
Sunday's march indicated that the anti-government activists may be shifting their tactics from peaceful demonstrations to confrontation. The protesters have massed for months in a large tent city protected by Gen. Ahmar's soldiers near Sanaa University. By leaving that area, and surging toward government ministries, the protesters quickly challenged troops.
"The protesters should not have marched to places guarded by security forces," said Mohammed Abdel-Malik Mutawakel, an opposition leader. But he blamed Mr. Saleh's soldiers for the death toll, saying, "Bloodshed from any side is completely wrong."
Government officials said the demonstrators were attempting to take over the state radio station and Cabinet building. Authorities said one soldier was killed and 65 were wounded. Protesters denied they were plotting to occupy government buildings.
Demonstrators were quickly overcome by tear gas and a steady barrage of bullets. Many were dragged away by friends after collapsing in the streets or were carried to makeshift hospitals. Bloodied bandages and ripped shirts littered floors and families waited on word about their sons and fathers.
Such scenes unfolded amid clashes between government troops and tribes increasingly hostile to Mr. Saleh. Sanaa in recent days has been shaken by explosions and gun fights, and the latest violence suggested that all parties in the conflict are growing more desperate.