Yemeni security forces opened fire on anti-regime demonstrators in Sanaa on Sunday, killing at least 26 and wounding hundreds more after lobbing mortar rounds at the home of a powerful dissident tribal chief.
Medics reported 26 dead and 500 wounded by live rounds, batons or after inhaling tear gas.
"Twenty six people were killed tonight," Tarek Nooman, a doctor in a field hospital in Sanaa's Change Square, epicentre of the anti-regime protests, told AFP.
Mohammed al-Abani, a doctor running another field hospital, said 500 people were hurt.
Witnesses said security forces and armed civilians opened fire on tens of thousands of protesters who left Change Square, where they have camped since February demanding regime change, and marched towards the city centre.
Water cannons and tear gas were also used, they added.
A medical official said that the injuries of 25 of those wounded by live rounds and shrapnel were critical.
Among them, he said, was a member of Yemen's national council, an umbrella of opposition groups, and a leading member of the Islamist Al-Islah (Reform) opposition party.
He named them as Mohammed al-Dhaheri, a professor of political science at Sanaa University, and Ahmed al-Qumairi.
Security forces later deployed heavily around Change Square, witnesses said, reporting exchanges of fire between First Armoured Brigade troops and forces loyal to President Ali Abdullah Saleh.
The interior ministry accused protesters of wounding four members of the security forces, throwing petrol bombs at electricity generators, and burning official vehicles.
It also accused the Common Forum, an alliance of the parliamentary opposition, of "pushing protesters towards staging armed rallies aimed at attacking public and private installations in an attempt to foil the dialogue."
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The national council accused Saleh's regime of committing a "massacre," saying it "strongly condemns this ugly crime that has killed dozens of Yemeni youths seeking freedom and who have faced... (the regime's) killing and destruction machine with bare chests."
It appealed to the United Nations and the international community "to take quick action and make practical decisions to end their silence and stop the crimes by this gang made up of the remains of Saleh's family regime," in a statement received by AFP.
The Organising Committee for the Youth Revolt in Sanaa insisted that its actions will remain "peaceful" and urged Yemenis to "come out and gather" across the country "day and night" until the regime falls.
It also asked for medical help and urged soldiers from the elite Republican Guard, headed by Saleh's son Ahmed, and the central security services led his nephew Yehya to "refuse to obey orders given to them by members of Saleh's family to kill their own people."
Saleh, who recovering in Saudi Arabia from wounds received in an explosion in Sanaa in June, last week authorised Vice President Abdrabuh Mansur Hadi to negotiate a power transfer with the opposition.
But the opposition has dismissed calls for dialogue before Saleh, in power since 1978, signs a Gulf-brokered deal that would see him hand power over to Hadi in return for amnesty from prosecution for himself and his family.
As clashes raged in the capital, massive demonstrations erupted in several cities south of Sanaa -- Taez, Yemen's second-largest city, Ibb, and Dhammar -- and in Saada in the north, to denounce the violence.
Witnesses in Taez said that police fired tear gas to disperse the protest there.
Earlier in Sanaa, heavy shelling targeted the area around the home of Sheikh Sadiq al-Ahmar, as his office and authorities exchanged blame over violence in the country.
Troops loyal to Saleh "opened fire using machine guns and are firing mortar rounds on the area surrounding Sheikh Sadiq al-Ahmar's home" in Al-Hasaba district, a source said from the tribal chief's home.
The shelling, which lasted two hours, also targeted Al-Mazda road in the district's centre, prompting people there to flee, witnesses said.
But Interior Minister Motahar Rashad al-Masri said that "Ahmar's gunmen, deployed on rooftops, opened fire on the ministry of interior and policemen who were having their lunch."
The troops "responded only to silence the source of fire," he added. "We are committed to self-restraint based on the orders of vice president" Hadi.
No casualties were reported.
Clashes between Ahmar's tribesmen and loyal troops in Al-Hasaba in May killed more than 300 people before a truce was agreed.