Fierce clashes broke out between rival security forces in the Yemeni capital Friday, two days after President Ali Abdullah Saleh inked a power transfer deal which sponsors had hoped would end the violence.
Dissident troops from the First Armoured Brigade led by General Ali Mohsen al-Ahmar exchanged mortar and machinegun fire in the early hours with forces of the central security services commanded by Saleh's nephew Yehya, residents said.
The clashes erupted outside the residence of Vice President Abrabuh Mansur Hadi, who is to assume Saleh's powers under the Gulf-brokered transition deal which the veteran strongman signed on Wednesday.
The fighting, which spread to the heart of the capital, lasted more than two hours. There was no immediate word on any casualties.
Tensions soared in the Al-Hasaba neighbourhood of north Sanaa -- scene of months of sporadic fighting between security forces and dissident tribesmen -- as rival gunmen set up roadblocks across the area.
Both Saleh's opponents and his supporters were expected to hold mass rallies in the capital later on Friday after the main weekly Muslim prayers.
Youth activists, who have spearheaded 10 months of protests against the president, have rejected the UN-backed transition deal, expressing outrage at its promise of immunity from prosecution for Saleh and his family, and demanding that his entire regime step down.
"Our revolt will continue until our aims are accomplished," the activists set as the slogan for Friday's protest.
Five people were killed when Saleh loyalists opened fire on a similar protest in Sanaa on Thursday.
Hundreds of people have been killed since protests against Saleh's 33-year rule erupted in January and human rights watchdog Amnesty International said the immunity pledge "deals a serious blow to victims of human rights violations."
Saleh, who was still in Saudi Arabia after Wednesday's signing of the Gulf transition plan, condemned the violence and ordered a probe.
The 69-year-old, who sustained serious blast wounds in a June bombing of his residence and has already received extensive treatment in Saudi Arabia, is to stay in Riyadh for medical tests, Yemeni Foreign Minister Abu Bakr al-Kurbi told the kingdom's Al-Watan daily.
"No specific date date has been set for his departure, as this depends on the results which will determine if he will be treated in the kingdom or in the United States," Kurbi told the newspaper.
"If the results are reassuring, he will return to Yemen."
UN chief Ban Ki-moon, whose Yemen envoy Jamal Benomar was instrumental in persuading Saleh to sign the Gulf transition plan after months of prevarication, had said he expected the president to travel to New York for treatment.
Under the deal, Saleh hands over, with immediate effect, all "necessary constitutional powers" to Vice President Abdrabuh Mansur Hadi and holds office on an honorary basis only for the coming 90 days.
His long equivocation over signing the blueprint, which the opposition first signed back in April, saw the protests slide into deadly clashes between loyalist and dissident troops and tribesmen that have riven the capital and left the armed forces deeply divided.
Besides Yehya, the president's son Ahmed commands the Republican Guards and Tariq, another nephew, controls the presidential guard.
But two major army divisions -- one in Sanaa and one in the second largest city Taez -- rallied to the opposition and have fought repeated battles against Saleh's loyalists, leaving scores dead.