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.
At least 19 people have now been killed in almost daily clashes at anti-regime protests since February 16, according to an AFP tally based on reports by medics and witnesses as calls gather steam for Saleh quit.
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.
They identified three of the dead as 17-year-old demonstrator Mohammed Ahmed Saleh, 21-year-old Hael Walid and Salem Bashaj, an employee at the state electricity company who was shot outside his home.
A hospital official in Aden confirmed to AFP that a fourth protester had died of wounds sustained in the gunfire.
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 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 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.
In the capital Sanaa, tens of thousands of protesters poured into a main square near Sanaa University chanting "Out, out!" and "God bears witness to your acts, Abdullah," an AFP correspondent reported.
Organisers estimated the numbers at 100,000.
"There is no solution unless the regime steps down," prayer leader Sheikh Abdullah Satar told the faithful over a megaphone.
Saleh loyalists also demonstrated in the capital's Tahrir square, where they have been since early February.
In Aden's Crater district, hundreds of demonstrators on Friday tried to attack a police station and blocked roads in other quarters with burning tyres, 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.
No casualties were reported.
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.
"The protesters carried banners reading 'No to oppression and tyranny,' 'Your blood, people of Sanaa, Taez and Aden has united Yemenis'," the statement said.