SANAA, Yemen (AP) - Yemeni government forces opened fire with anti-aircraft guns and automatic weapons on tens of thousands of anti-government protesters in the capital demanding ouster of their longtime ruler, killing at least 26 and wounding dozens, medical officials and witnesses said.

After nightfall, Sanaa sank into complete darkness after a sudden power outage.

The attack was the deadliest in months against protesters and comes as tensions have been escalating in the long, drawn-out stalemate between the regime and the opposition. The president, Ali Abdullah Saleh, left for Saudi Arabia for treatment after being severely wounded in a June 3 attack on his palace, raising hopes for his swift removal - but instead, he has dug in, refusing to step down.

The protest movement has stepped up demonstrations the past week, angered after Saleh deputized Vice President Abed Rabbo Mansour Hadi 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 the powerful Republican Guards force, led by Saleh's son and heir apparent Ahmed and armed regime supporters have also been turning out in the streets in recent days, raising fears of a new bloody confrontation.

More than 100,000 protesters massed Sunday around the state television building and government offices, witnesses said. When the crowd began to march toward the nearby Presidential Palace, security forces opened fire, they said. Snipers fired down at the crowd from nearby rooftops, and plainclothes Saleh supporters armed with automatic rifles, swords and batons attacked the protesters. Protesters took control of a main bridge, closed off the entrances and set fire to tents in a camp used by pro-government forces.

"This peaceful protest was confronted by heavy weapons and anti-aircraft guns," said Mohammed al-Sabri, an opposition spokesman. He vowed that the intensifying protests "will not stop and will not retreat."

At the neighborhood of al-Zubairi in the heart of Sanaa, troops opened fire at an anti-government force, the 1st Armored Division led by Maj. Gen. Ali al-Ahmar, who defected to the opposition along with his 50,000 troops several months ago.

Witnesses said al-Ahmar's forces engaged in the fighting Sunday for the first time, but Abdel-Ghani al-Shemari, spokesman for al-Ahmar division denied that and said they are "maintaining self-restraint."