After nightfall, Sanaa sank into complete darkness after a sudden power outage, as protesters took control of a vital bridge, halting traffic and setting up tents.
Thousands of other protesters attacked government buildings and set fires to buildings they said were used by snipers and pro-government thugs.
Sunday's 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.
More than 100,000 protesters massed Sunday around the state radio building and government offices, witnesses said.
When the crowd began to march toward the nearby Presidential Palace, security forces opened fire and shot tear gas canisters, 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.
Tarek Noaman, a doctor at Sanaa field hospital, said that 26 protesters were shot dead and more than 200 were wounded.
"Most of the injuries are at the chest, shoulder, head and face," he said, and said 25 of injured protesters were in critical condition.
He accused security forces of preventing ambulances from evacuating the wounded and collecting bodies of the slain protesters.