Yemeni security forces opened fire on anti-regime demonstrators in Sanaa yesterday, killing at least 20 and wounding hundreds more after firing mortar rounds near the home of a powerful dissident tribal chief.

Medics in the capital reported 20 dead and 500 others wounded by live rounds, batons or suffering from breathing difficulties after inhaling tear gas.

“Twenty people have been killed – four of them (were declared dead) at the Sciences and Technology hospital and the remaining 16 at a field hospital,” a medic said.

Earlier Mohammed al-Abani, who heads a Sanaa field hospital, reported 12 dead and said: “Five hundred others were wounded by live rounds and batons or suffer from breathing difficulties due to inhaling tear gas.”

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.

They also used water cannons and fired tear gas, they added.

A medical official said that the injuries of 25 of those wounded by live rounds and shrapnel were critical.

Among those, he said, were members of Yemen’s national council, an umbrella of opposition groups.

He named them as Mohammed al-Dhaheri, a professor of political science at Sanaa University, and Ahmed al-Qumairi of the Islamist Al-Islah (Reform) opposition group.

The interior ministry accused protesters of wounding four members of the security forces, throwing petrol bombs at electricity generators, and burning one police car and one firefighting vehicle, in statements on state television.

It also accused the Common Forum, an alliance of parliamentary opposition, of “pushing protesters towards staging armed rallies aimed at attacking public and private installations in an attempt to foil the dialogue.”

Last week embattled President Ali Abdullah Saleh, who has been absent from Yemen for more than three months, 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 his deputy in return for amnesty from prosecution for himself and his family.