|Opposition plans to hold a "Friday of Honouring Martyrs" rally, while a pro-government rally is also expected [AFP]
Yemeni opposition has warned the government that violence against pro-democracy protesters could derail a deal aimed at ending the political standoff.
An opposition bloc on Thursday accused embattled President Ali Abdullah Saleh's regime of "massacring" peaceful protesters in a bid to derail a Gulf-led transition plan.
Plainclothes gunmen killed 12 people and wounded dozens more in Yemen's capital on Wednesday, when they opened fire on anti-government marchers, just days before a Gulf-mediated deal to end the crisis was due to be sealed.
"In the event of your inability to protect protesters, we will find ourselves unable to pursue an agreement that the regime seeks to use to shed more blood," an opposition coalition said in a statement.
Tens of thousands of people rallied across Yemen on Thursday, denouncing the deaths.
Meanwhile, rival rallies have been planned for Friday in Sanaa, the capital. The government planned a "Friday of Constitutional Legitimacy", while the opposition planned a "Friday of Honouring Martyrs".
A deal to end the crisis by easing Saleh out within a month was expected to be signed on Sunday in Saudi capital Riyadh, three months after Yemenis took to the streets, inspired by revolts that toppled autocratic rulers in Egypt and Tunisia.
But on Thursday, Saleh appeared to raise a potential problem when he told Russia's Arabic language TV that he objected to the presence of Qatari representatives.
"We will have reservations about signing if representatives of Qatar are present among the Gulf foreign ministers," Saleh told Russia Today. "(Qatar) is involved in a conspiracy not just against Yemen but against all Arab countries."
He singled out Qatar-based Al Jazeera channel, which Saleh has accused in the past of provoking the protests.
The balance of power has tipped against Saleh, long a key ally of the West against al-Qaeda, after weeks of violence, military defections and political reversals.
In a related incident on Friday, the Yemeni president has fired the country's attorney general.
Saleh to sign deal
Washington and neighbouring oil producer Saudi Arabia want the standoff resolved to avert a descent into more bloodshed in the Arabian Peninsula state that would offer more room for a Yemen-based al-Qaeda wing to operate.
The deal, brokered by the six-member Gulf Cooperation Council, would give Saleh and his family and aides immunity from prosecution.
It provides for Saleh to appoint a prime minister from the opposition, who would then form a transition government ahead of a presidential election two months after his resignation.
A sizable section of protesters are against giving Saleh one month time to quit fearing it may offer time for potential sabotage.
Whoever leads Yemen's transitional government will not only struggle to quash an aggressive al-Qaeda branch, which has tried to hit US and Saudi targets, but also inherit simmering rebellions in the north and south of the country.
The US embassy in Sanaa said the upsurge in violence on the eve of a power transfer deal was "especially disturbing" and urged all sides to act with restraint.
At the same time, Ban Ki-moon, UN secretary-general, said he was concerned by the deadly violence, reiterating a call on authorities to protect civilians and welcoming efforts towards a peaceful transition, his spokesman said.
Ban appealed "to all concerned in Yemen to exercise utmost restraint and desist from provocative acts."
Human rights group Amnesty International called for an urgent and independent investigation into Wednesday's killings as protesters marched past a stadium.
"Men who are believed to have been members of the security forces or militant government supporters, reportedly fired from rooftops and from inside the stadium," it said.