Clashes between the Syrian army and rebel fighters have broken out for the first time since a hard-won UN-backed ceasefire came into force a day earlier, a monitoring group says.
"Fighting with heavy machineguns took place in Khirbet al-Joz, located on the Turkish border, between regime soldiers and (army) deserters," Rami Abdel Rahman of the Syrian Observatory for Human Rights told AFP in Beirut on Friday.
Syria's opposition has also called for major protests on Friday, in what would be the first real test of the shaky ceasefire to end a 13-month crackdown on dissent that has killed thousands.
The ceasefire had appeared to be holding for a second day, after world powers called for swift action to send in UN observers to monitor the fragile truce.
Violence on Thursday killed at least eight people, including seven civilians, and wounded dozens more, said the Observatory, a Britain-based monitoring group.
Even so, the toll is markedly lower than it has been in recent weeks, when there have often been scores of people killed.
After the ceasefire came into force at dawn on Thursday, peace envoy Kofi Annan declared he was "encouraged by reports that the situation in Syria is relatively calm and that the cessation of hostilities appears to be holding".
But as the government of Syrian President Bashar al-Assad and the rebels traded accusations of trying to wreck the ceasefire, Annan insisted that "all parties have obligations to implement fully the six-point plan".
"What has happened today does not constitute full compliance by the Syrian government," the UN-Arab League envoy was quoted as saying Thursday. "Syrian troops and armour must return to their barracks immediately."
Annan's plan calls for the withdrawal of forces from urban areas, the release of arbitrarily detained people, freedom of movement for journalists and the right to demonstrate.
Despite the regime's commitment to pull back, the spokeswoman for the opposition Syrian National Council (SNC), Basma Qoudmani, said "we have concrete proof that heavy weapons are still in population centres".
The SNC, the most widely recognised opposition group in exile, and internet-based activists called for peaceful demonstrations across Syria to test the government's readiness to accept public shows of dissent.
"We call on the people to demonstrate and express themselves... The right to demonstrate is a principle point of the plan," Burhan Ghalioun, head of the SNC, told AFP.
Qoudmani said: "The real test (of the ceasefire) will be if there is shooting or not when people demonstrate."
The Syrian Revolution 2011 activist group also called on its Facebook page for protests on Friday - the Muslim day of rest when the demonstrations have been the largest - under the rallying cry "A revolution for all Syrians".
But Syria's interior ministry insists people wanting to demonstrate must have permits.
"The right to demonstrate peacefully is guaranteed by law. We call on citizens to apply the law by requesting a permit before demonstrating," said a statement carried by the official SANA news agency.
The ministry said the measure is aimed at "securing the safety of citizens and to practise this right in a civilised manner".
On Friday, the UN Security Council could vote on a resolution authorising the deployment of UN observers to monitor both sides to the conflict in Syria, which monitors say has cost more than 10,000 lives since March 2011.
UN chief Ban Ki-moon said plans were being drawn up to send observers to Syria, starting with the dispatch of a UN peacekeeping general as early as Friday.
An advanced mission of 20-30 observers could be in place early next week, diplomats said. The full mission would be at least 200 monitors.
Ban said "the world is watching however with sceptical eyes", adding that previous promises made by the regime "have not been kept".
World powers said the tentative truce in Syria was a "fragile" first step and joined the calls for Damascus to carry out a broader peace plan and permit international observers to monitor it.
In a statement after two days of talks in Washington, foreign ministers from the Group of Eight major economies, which include Western powers and Syria's main supporter Russia, urged "immediate" action to send in observers.
French President Nicolas Sarkozy said on Friday he did not believe in the sincerity of Syrian leader Bashar al-Assad, nor in the ceasefire.
"I do not believe in Bashar al-Assad's sincerity, nor unfortunately in the ceasefire. I think we must deploy observers so that at the very least we know what is happening," he said in an interview with French television I-Tele.
Syria's government urged tens of thousands of people who fled the violence both inside and outside the country to return home and offered an amnesty to opposition gunmen without "blood on their hands".
The rebel Free Syrian Army, for its part, insisted it was sticking to the ceasefire.
"The regime is being elusive. We are 100 per cent committed to the ceasefire, but the regime is not abiding by it," said FSA spokesman Colonel Kassem Saadeddine.