Amnesty International said at least 11 anti-government protesters and bystanders were killed.
Meanwhile, suspected Al-Qaeda gunmen on a motorbike shot dead an intelligence officer in the town of Shahr, in southeastern Hadramaut province, a security official told AFP.
Powerful tribal leaders, including those of the Hashid and Baqil, pledged to join protests against Saleh at a gathering north of the capital, a tribal source said.
"I have announced my resignation from the General People's Congress in protest at the repression of peaceful demonstrators in Sanaa, Taez and Aden," Hashid tribe chief Sheikh Hussein bin Abdullah al-Ahmar was quoted as saying in reference to the ruling party.
The Hashids are considered Yemen's most powerful tribal confederation and include nine clans, among them the Sanhan, long a bulwark of Saleh's regime.
The announcement was warmly received by a large crowd of tribesmen, including members of Yemen's second largest tribe, the Baqil, who gathered for the meeting, said the tribal source.
The two tribes announced they would support the popular uprising against Saleh, who has refused to step down after three decades in power, to chants of "the people want the fall of the regime!"
According to an AFP tally based on reports by medics and witnesses, at least 19 people have been killed in almost daily clashes at protests since February 16. Amnesty International puts the death toll at 27.
Medics said security forces used live ammunition on a demonstration in the southern city of Aden, which has seen the worst violence, bringing the death toll to four on Saturday from just one rally with 40 others wounded.
They identified three of the dead as demonstrator Mohammed Ahmed Saleh, 17; Hael Walid, 21, and Salem Bashaj, an employee at the state electricity company who was shot outside his home.
A hospital official in Aden told AFP that a fourth protester died of wounds sustained in the gunfire, which came after Saleh said he had ordered his forces to protect both pro- and anti-government demonstrators.
Amnesty said in a statement issued Saturday that it had received reports the security forces refused to allow the wounded to be taken to hospital.
"Events in Yemen are taking a serious turn for the worse and the Yemeni security services are showing reckless disregard for human life," said Philip Luther, the rights watchdog's director for the Middle East and North Africa.
Residents of Aden described Friday night's shooting as a scene from the frontlines of all-out war.
"Our neighbourhood has witnessed real scenes of war waged by forces of the Republican Guard, who have been targeting our innocent young who want to protest peacefully," one resident told AFP.
News of 17-year-old Saleh's death sparked a wave of anger in neighbourhoods across Aden, where residents attacked police stations and set fire to a police car, an AFP correspondent said.
Aden remained tense on Saturday as a handful of families searched for missing relatives whom they believed were arrested.
But a Yemeni official denied reports of a police raid.
"An armed separatist group loyal to the so-called Hirak (Southern Movement) randomly opened fire with automatic weaponry on buildings in the district of Maalla, targeting security forces and citizens," the defence ministry's 26sep.net website quoted the security official as saying.
Saleh has stubbornly refused to resign, but said he will not seek re-election in 2013 and promised political reforms.
A Friday demonstration, dubbed "the beginning of the end" of his regime which swept to power in 1978, saw 100,000 Yemenis turn out across the country, organisers said.
Pro- and anti-government demonstrators have also clashed in recent days, with Saleh loyalists locking horns with militants from the Southern Movement in the country's south.
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Meanwhile, suspected Al-Qaeda gunmen on a motorbike shot dead an intelligence officer in the town of Shahr, in southeastern Hadramaut province, a security official told AFP.
Powerful tribal leaders, including those of the Hashid and Baqil, pledged to join protests against Saleh at a gathering north of the capital, a tribal source said.
"I have announced my resignation from the General People's Congress in protest at the repression of peaceful demonstrators in Sanaa, Taez and Aden," Hashid tribe chief Sheikh Hussein bin Abdullah al-Ahmar was quoted as saying in reference to the ruling party.
The Hashids are considered Yemen's most powerful tribal confederation and include nine clans, among them the Sanhan, long a bulwark of Saleh's regime.
The announcement was warmly received by a large crowd of tribesmen, including members of Yemen's second largest tribe, the Baqil, who gathered for the meeting, said the tribal source.
The two tribes announced they would support the popular uprising against Saleh, who has refused to step down after three decades in power, to chants of "the people want the fall of the regime!"
According to an AFP tally based on reports by medics and witnesses, at least 19 people have been killed in almost daily clashes at protests since February 16. Amnesty International puts the death toll at 27.
Medics said security forces used live ammunition on a demonstration in the southern city of Aden, which has seen the worst violence, bringing the death toll to four on Saturday from just one rally with 40 others wounded.
They identified three of the dead as demonstrator Mohammed Ahmed Saleh, 17; Hael Walid, 21, and Salem Bashaj, an employee at the state electricity company who was shot outside his home.
A hospital official in Aden told AFP that a fourth protester died of wounds sustained in the gunfire, which came after Saleh said he had ordered his forces to protect both pro- and anti-government demonstrators.
Amnesty said in a statement issued Saturday that it had received reports the security forces refused to allow the wounded to be taken to hospital.
"Events in Yemen are taking a serious turn for the worse and the Yemeni security services are showing reckless disregard for human life," said Philip Luther, the rights watchdog's director for the Middle East and North Africa.
Residents of Aden described Friday night's shooting as a scene from the frontlines of all-out war.
"Our neighbourhood has witnessed real scenes of war waged by forces of the Republican Guard, who have been targeting our innocent young who want to protest peacefully," one resident told AFP.
News of 17-year-old Saleh's death sparked a wave of anger in neighbourhoods across Aden, where residents attacked police stations and set fire to a police car, an AFP correspondent said.
Aden remained tense on Saturday as a handful of families searched for missing relatives whom they believed were arrested.
But a Yemeni official denied reports of a police raid.
"An armed separatist group loyal to the so-called Hirak (Southern Movement) randomly opened fire with automatic weaponry on buildings in the district of Maalla, targeting security forces and citizens," the defence ministry's 26sep.net website quoted the security official as saying.
Saleh has stubbornly refused to resign, but said he will not seek re-election in 2013 and promised political reforms.
A Friday demonstration, dubbed "the beginning of the end" of his regime which swept to power in 1978, saw 100,000 Yemenis turn out across the country, organisers said.
Pro- and anti-government demonstrators have also clashed in recent days, with Saleh loyalists locking horns with militants from the Southern Movement in the country's south.
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