Syrian tanks fired on opposition districts in Homs on Friday, killing four people, activists said, ahead of a mission by UN-Arab League envoy Kofi Annan to end a year-long conflict edging into civil war.
Opponents of President Bashar al-Assad said they were planning to show their strength in the streets after weekly Muslim prayers, but the tankfire kept many indoors in Homs.
They said nationwide protests would mark the anniversary of Kurdish unrest in northeastern Syria in 2004 that was crushed by security forces with about 30 people killed.
Annan has called for dialogue to reach a political solution, but opposition figures chided him for a proposal they said would only give Assad's forces more time to crush his foes.
Rifts among big powers have blocked any UN action to resolve the crisis, with China and Russia firmly opposing any measure that might lead to Libya-style military intervention.
China welcomed the former UN chief's mission. "We hope that Mr Annan uses his wisdom and experience to push for all sides in Syria to end their violence and start the process of peace talks," Foreign Ministry spokesman Liu Weimin said.
China, which dispatched an envoy to Syria this week, said on Friday it would send an assistant foreign minister to the Middle East and to France to discuss the crisis. Beijing has told other powers not to use humanitarian aid to "interfere" in Syria.
Russia, an old ally of Damascus and its main arms supplier, has defended Assad against critics of his bloody crackdown, twice joining China in vetoing UN resolutions on Syria.
"We shall not support any resolution that gives any basis for the use of force against Syria," Russian Deputy Foreign Minister Gennady Gatilov tweeted late on Thursday.
A Russian diplomat said Assad was battling al Qaeda-backed "terrorists" including at least 15,000 foreign fighters who would seize cities if government troops withdrew.
Moscow could play a vital role in any diplomatic effort to ease Assad from power and spare Syria further bloodletting.
"If (Annan) can persuade Russia to back a transitional plan, the regime would be confronted with the choice of either agreeing to negotiate in good faith or facing near-total isolation through loss of a key ally," the Brussels-based International Crisis Group said in a paper this week.
In Damascus, security forces killed a civilian overnight when residents of the Kfar Souseh district shouted "Allahu Akbar" (God is great) from their homes, activists said.
The funeral of Mohammad Sarayji, 30, was due to take place at noon, an occasion likely to lead to further protests.
The state news agency SANA said "huge crowds" had turned out in Damascus and the northeastern city of Hassaka to show support for Assad's reforms and opposition to "foreign interference".
Syrian security forces have already killed well over 7,500 people since the anti-Assad uprising began a year ago, according to a UN estimate. The government said in December that "armed terrorists" had killed more than 2,000 soldiers and police.
The British-based Syrian Observatory for Human Rights reported 18 more killed on Thursday, while the grassroots Local Coordination Committees put the death toll at 62, including 44 people it said had been slain in cold blood in Homs.
UN humanitarian chief Valerie Amos was visiting Syrian refugees in border camps in Turkey after her trip to Syria, where she found scenes of destruction in Homs but few civilians in the battered former rebel bastion of Baba Amr.
Annan has an impressive record as a mediator, but he will need all his skills to halt Syria's slide into civil war.
"The killing has to stop and we need to find a way of putting in the appropriate reforms and moving forward," he said in Cairo where he met Arab League chief Nabil Elarabi. Annan said any further militarization of the conflict would make it worse.
Syrian dissidents reacted with dismay and said government repression had destroyed prospects of a negotiated deal.
"We reject any dialogue while tanks shell our towns, snipers shoot our women and children and many areas are cut off from the world by the regime without electricity, communications or water," said Hadi Abdullah, an activist contacted in Homs.
The Syrian opposition denies any al Qaeda role in a popular uprising against nearly five decades of Baathist rule that erupted last March, inspired by Arab revolts elsewhere.
Assad's violent response to peaceful demonstrations prompted some Syrians, mostly lightly armed Sunni Muslim army deserters, to fight back in an unequal struggle against Assad's 300,000-strong military, secret police and feared Alawite militiamen.
Two rebel groups said four brigadier-generals had defected over the past three days to a camp for Syrian army deserters in Turkey. Earlier Syria's deputy oil minister became the first senior civilian official to announce his defection.