Two people killed and ten injured by security gunfire in the Yemeni city of Taez, Al Arabiya reported.
Sunday's violence, jolting an uneasy, weeks-long stalemate, was the worst in recent months. Hundreds of people in an anti-government march were wounded as well when security forces fired on protesters who charged police lines.
Opposition organizers called for more action on Monday, rousing sleeping protesters who have been camped in Sanaa's Change Square for eight months to demand an end to 33 years of autocratic Saleh rule in impoverished Yemen.
Shouting over loudspeakers, organizers urged protesters to head back to a junction they had taken on Sunday night, known locally as Kentucky roundabout, and hinted they planned to push further into territory held by government forces.
The area had previously marked the dividing line between the parts of Sanaa held by loyalist troops and defected forces.
"Come on everyone, we will have breakfast at Kentucky roundabout and later we will have lunch somewhere else further down!" the speaker shouted.
But blocking the way of protesters trying to move forward were troops belonging to defected General Ali Mohsen, who threw his support behind the anti-Saleh movement some months ago but now seemed to be trying to defuse the situation.
In Geneva on Monday, Yemeni Foreign Minister Abubakr Abdullah Al-Qirbi said Sunday's bloodshed would be investigated and perpetrators would be prosecuted.
In a speech to the U.N. Human Rights Council, he said: "The government of Yemen expresses its sorrow and condemnation for all acts of violence and bloodshed as those that happened yesterday in Sanaa. The government will investigate and hold accountable all those in charge of these acts."
Sanaa for months has been divided between Mohsen's defected troops and loyalist forces in a maze of checkpoints, roadblocks and armored vehicles that many worry could quickly tip inflamed tensions into military confrontation.
Protesters on Monday managed to extend the territory of their camp by around one km, and hundreds slept there overnight. Ali Mohsen's troops entered the area and were fortifying it with sandbags.
The new staked-out area brought protesters and the defected troops backing them within 500 meters of Ahmed Ali Saleh, the president's son and head of the Republican Guard units loyal to the government.
Yemen for months has been mired in a political stalemate as Saleh, who is currently being treated in Saudi Arabia after an June assassination attempt, clings to power despite mass protests across the country.
Unrest extended to the south of Yemen as well.
In Taiz, another hotbed of anti-government protests, opposition sources said there was heavy shelling overnight by security forces after they too held large rallies on Sunday, Reuters reported.