At least one anti-regime protester was killed and dozens shot in the cities of Taez and Sanaa, as Yemen's worried Gulf neighbours prepared to meet in Riyadh on Sunday to work out a transition plan.
Three other people were killed, including an intelligence officer and a soldier, in Abyan province, an al-Qaeda stronghold in southern Yemen.
The protester died of a gunshot wound after security forces using live fire hit dozens of demonstrators in overnight clashes in Sanaa and Taez, a city to the south of the capital, medics said.
"One protester died of his wounds late on Saturday," said a medic treating casualties at a makeshift field hospital in the flashpoint city of Taez.
More than 40 protesters were wounded by live gunfire in clashes between security forces and demonstrators calling for President Ali Abdullah Saleh's ouster that raged deep into Saturday night, medics said.
The latest protests were fuelled by anger after four demonstrators were shot dead in Taez on Friday. A week earlier, 17 demonstrators were gunned down in the city in clashes with security forces on April 3-4, according to medics.
In Sanaa, security forces shot and wounded 30 people on Saturday, while 80 others suffered injuries from beatings with batons, medics and demonstrators said.
Another 1,200 people needed treatment for tear-gas inhalation, said members of a medical committee formed by protesters who have staged a sit-in at a square near Sanaa University since February.
Tens of thousands rallied in the capital again on Sunday, chanting anti-regime slogans and demanding that Saleh step down. Witnesses said they kept their distance from police control points to prevent fresh clashes.
Foreign ministers of the six-nation Gulf Cooperation Council were to meet in the Saudi capital later on Sunday to discuss the unrest in Yemen, the impoverished neighbour of the region's oil-rich monarchies.
The meeting was going ahead despite tensions between Yemen and Qatar, a GCC member which has called for Saleh to quit and allow a transition.
On Friday, Saleh telephoned GCC leaders, Yemen's embassy to Washington said, listing all the calls, with Qatar's Emir Hamad bin Khalifa al-Thani the only name missing.
Qatar's prime minister said the previous day that GCC countries "hope to reach a deal with the Yemeni president to step down," a position Saleh slammed as a case of "blatant interference in Yemeni affairs."
Yemen responded by recalling Sanaa's ambassador to Doha.
In other violence on Saturday, an intelligence officer was killed in a drive-by shooting outside his home in the Loder area of Abyan, doctors and security sources said.
His son was also wounded when two men on a motorbike opened fire on Colonel Hussein Gharama and sped off, according to witnesses, who identified the assailants as al-Qaeda members.
And a soldier and a militant were killed in a clash between security forces and suspected al-Qaeda fighters in Jaar, also in Abyan, a local security official told AFP.
Army artillery pounded a suspected al-Qaeda hideout in Jaar for several hours on Saturday after the military urged local residents to evacuate the area. Residents later returned to their homes after the guns went silent.
A source close to the militants told AFP that the army had withdrawn, and that two of its gunmen had been wounded in the shelling.
The United States, which has counted on Saleh in its battle against al-Qaeda, on Friday urged all sides in Yemen to engage in an "urgently needed dialogue" on a political transition.
Washington has expressed concern that al-Qaeda militants based in lawless regions of Yemen have taken advantage of the political unrest as Sanaa eased the pressure on the Islamists.