Pressure on Ali Abdullah Saleh to resign intensified on Saturday when the leaders of two of Yemen's most important tribes abandoned the president and joined the anti-regime movement.
The news came as an official denied reports that police killed four people on Friday in an assault on an anti-government protest in Aden, blaming a southern secessionist group for the attack.
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 told AFP.
"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 tribal 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, according to 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!"
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 tribal chief Sheikh Hussein bin Abdullah al-Ahmar
Four people killed
Police gunfire killed four people as the biggest protests yet swept impoverished Yemen, demanding that President Ali Abdullah Saleh step down after three decades in power.
Twenty-one people have now been killed since Feb. 17 in daily protests against Saleh's 32-year rule. Unrest has been especially intense in the once-independent south, where many people resent rule from the north.
Inspired by similar revolts that toppled the presidents of Tunisia and Egypt, the latest killings came just one day after Saleh ordered his forces to offer "full protection" to protesters and loyalists alike.
He has stubbornly refused to resign, saying he will not seek re-election when his current term ends in 2013 and promising political reforms.
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, medics said.
Residents of the Aden district of Maalla, where the raid happened, have begun to fear their city has become the frontline in a state-sponsored war.
"Our neighborhood 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 on condition of anonymity.
Security forces also arrested many demonstrators, residents told AFP.
Clashes between police and protesters on Friday continued through the night. Witnesses said security forces opened fire at a sit-in outside a hotel on the main street in Maalla to demand the departure of Saleh.
Yemen's protesters dubbed Friday "the beginning of the end" for Saleh's regime, which swept to power in 1978.
Loyalists and opponents of the 68-year-old leader held rival demonstrations in the capital Sanaa after Friday prayers.
Around 100,000 thousand anti-Saleh protesters shouted slogans which have echoed around the Arab world since the uprisings in Egypt and Tunisia: "The people demand the downfall of the regime."
Saleh supporters chanted their loyalty to a man they see as holding the impoverished tribal nation together. "The creator of unity is in our hearts. We will not abandon him," they cried.
In Aden's Crater district, hundreds of demonstrators on Friday tried to attack a police station and blocked roads in other quarters with burning tires, witnesses said.
Protesters were also prevented from gathering in a square near the security headquarters and several consulates.
Anti-Saleh demonstrations have spread across the country, often clashing with loyalists to the regime.
In the south, pro-Saleh demonstrators also clashed in Hadramawt with militants from the Southern Movement carrying banners calling for the secession of the formerly independent south, witnesses said.
And in Yemen's northern Saada province, "tens of thousands demonstrated" against Saleh and in solidarity with protesters across the country, Shiite rebels who have fought an on-off uprising against the regime since 2004 said in a statement on their website almenpar.net.