Gunfire and shelling rocked Sanaa for a third day Tuesday, killing 23 people and taking the toll from the worst outbreak of violence in the Yemeni capital in months to 76 before a tenuous truce took hold.
In reaction, the United States called for an end to the violence and for a prompt political settlement.
Fighting between dissident troops and those loyal to embattled President Ali Abdullah Saleh resumed at dawn after a brief lull overnight and raged through the day before receding in the evening, medics and witnesses said.
The defence ministry said early evening that Vice President Abdrabuh Mansur Hadi had given "strict orders for a rapid ceasefire in the capital and that government forces were obeying."
An opposition official said the dissident troops had been observing a ceasefire since noon to "foil the plans of the band that wants a military escalation."
EU foreign policy chief Catherine Ashton had earlier urged an immediate end to the bloodshed and pressed Saleh to agree to a transfer of power in the face of the mass protests that have rocked his regime since mid-January.
In Washington, State Department spokeswoman said "the United States continues to support the Yemeni people's aspirations for a peaceful and orderly transition that is responsive to their aspirations for peace, reconciliation, prosperity and security.
"A political solution is the best way to avoid further bloodshed," she said while attending meetings with Secretary of State Hillary Clinton on the sidelines of the UN General Assembly in New York.
"We remain hopeful that an agreement will be reached that leads to the expeditious signing of the GCC (Gulf Cooperation Council) political transition initiative."
Republican Guard troops, commanded by Saleh's son, Ahmed, shelled posts held by troops of the First Armoured Brigade loyal to dissident General Ali Mohsen al-Ahmar around Change Square, epicentre of the protests, witnesses said.
Change Square was targeted by mortar rounds and anti-aircraft fire, with one witness describing it as the "heaviest shelling" yet and saying it "lit the sky over the square."
Protest organisers told AFP the numbers of demonstrators camped in an area stretching about three kilometres (two miles) from Change Square to Al-Zubair Road had swelled to nearly 150,000.
Meanwhile, Valerie Petitpierre, deputy head of the International Committee of the Red Cross delegation in Yemen, spoke of "receiving very worrying reports of armed confrontations taking place in Al-Gomhori Hospital and placing many innocent lives at risk.
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In addition, "over the past three days, Yemen Red Crescent Society emergency response teams have been threatened and assaulted," said Petitpierre.
Ashton's spokesman said in Brussels that she "deplores the many deaths and injuries following the demonstrations in Sanaa on 18-19 September," adding that perpetrators of violence should be held accountable.
"In this volatile situation, it is crucial to exercise restraint, avoid provocative action, refrain from further violence and take immediate steps to de-escalate the situation," the statement said.
"The EU reiterates the need to sign and implement the Gulf Cooperation Council initiative and stands ready to extend support to Yemeni stakeholders, the GCC and our international partners."
Saleh, who has ruled Yemen since 1978, has been recovering in Saudi Arabia after a June 3 explosion at his presidential compound, but has so far refused to transfer power to his deputy or to sign the so-called Gulf Initiative.
The GCC plan, proposed last spring, calls on Saleh to step down as president and hand over all constitutional authorities to his deputy. In return, Saleh and his family would be granted immunity from prosecution.
However, young protesters reject the plan and call for the immediate ouster of Saleh and all members of his regime and for bringing him to justice.
The latest violence was sparked on Sunday when demonstrators, vowing to escalate their protests, marched southward along Al-Zubair Road towards Kentaky crossroad in central Sanaa.
They were met by security forces and armed civilians who opened fire on them, leaving 26 dead, medics said.
Dissident troops intervened in defence of the protesters, who managed to drive out Saleh's loyalists and security forces before setting up their own tents along the road.
On Monday, 27 people were killed in similar clashes, medics and protest organisers said.
Five civilians were also killed in the city of Taez south of Sanaa while six others were wounded in random shelling and gunfire by Saleh's forces, according to medics and residents there.
The bloodletting coincides with the arrival in Sanaa of UN envoy to Yemen Jamal Benomar and GCC chief Abdulatif al-Zayani for what a diplomat said was the signing of a UN roadmap for the transfer of power.
The opposition has declined to meet any of the officials. "The opposition cannot receive them while blood is flowing in Sanaa," said a leading activist.
But the deputy leader of the ruling General People's Congress, Sultan al-Barakani accused the opposition of having "hindered" mediation efforts and said protesters had to return to their "previous positions" in Change Square.