BEIRUT (AP) — Syria’s opposition called for widespread protests Friday to test the regime’s commitment to an internationally brokered cease-fire that the U.N. chief described as so fragile it could collapse with a single gunshot.

Regime forces halted heavy shelling and other major attacks in line with the truce that began at dawn Thursday, though there were accusations of scattered violence by both sides. The government ignored demands to pull troops back to barracks, however, defying a key aspect of the plan, which aims to calm a year-old uprising that has killed 9,000 people and has pushed the country toward civil war.

“The onus is on the government of Syria to prove that their words will be matched by their deeds at this time,” U.N. Secretary-General Ban Ki-moon told reporters in Geneva. He said the world was watching with skeptical eyes.

“This cease-fire process is very fragile. It may be broken any time,” Ban added, saying “another gunshot” could doom the truce.

The presence of tanks and troops could discourage any large gatherings, but the leader of the opposition Syrian National Council, Burhan Ghalioun, urged Syrians to demonstrate peacefully on Friday. “Tomorrow, like every Friday, the Syrian people are called to demonstrate even more and put the regime in front of its responsibilities – put the international community in front of its responsibilities.”

A massive protest would be an important test of the cease-fire – whether President Bashar Assad will allow his forces to hold their fire and risk ushering in a weekslong sit-in or losing control over territory that government forces recently recovered from rebels.

So far, the military crackdown has prevented protesters from recreating the powerful displays of dissent seen in Egypt’s Tahrir Square, where hundreds of thousands of people camped out in an extraordinary scene that drove longtime leader Hosni Mubarak from power.

If the truce holds, it would be the first time the regime has observed an internationally brokered cease-fire since Assad’s regime launched a brutal crackdown 13 months ago on mass protests calling for his ouster.

“The test will come when we start to see protests across the length and breadth of the country,” said Salman Shaikh, director of the Brookings Doha Center. “Is the Assad regime willing to accept that there will likely be hundreds of thousands of people on the streets in the next few days? And will they accept those protesters, if they are not breaking any laws, occupying certain spaces and towns and centers of towns, should that start to arise?”

An outbreak of violence at a chaotic rally could give the regime a pretext for ending the truce. And it would be difficult to determine the source of such an attack, given that Syria is largely sealed off from journalists and outside observers.

The U.N. chief’s envoy, Kofi Annan, urged the 15-nation U.N. Security Council to authorise an observer mission that would keep the cease-fire going and to demand that Assad order his troops back to barracks, U.N. diplomats said. The council could adopt a resolution on the observers as early as Friday, the diplomats said on condition of anonymity because the meeting was closed.

Western powers, skeptical that Assad will call off the killings, said an end to violence is just the first step.

U.K. Prime Minister David Cameron urged Syria’s allies Russia and China to help “tighten the noose” around Assad’s regime. Russia and China have blocked strong action against Syria at the Security Council, fearing it would open the door to possible NATO airstrikes like those that helped topple Libya’s Moammar Gadhafi.

U.S. Secretary of State Hillary Rodham Clinton noted that Assad failed to comply with key obligations, such as pulling back tanks.

“The burden of fully and visibly meeting all of these obligations continues to rest with the regime,” she said. “They cannot pick and choose. For it to be meaningful, this apparent halt in violence must lead to a credible political process and a peaceful, inclusive democratic transition.”

The U.S. Embassy in Damascus published an image on its Facebook page that purports to show tanks deployed within the city of Homs.

“Clearly, Assad is not complying,” the embassy said.

Bassma Kodmani, a spokeswoman for the opposition Syrian National Council, said a heavy security presence, including checkpoints and snipers, remained in the streets despite the cease-fire.

“There is no evidence of any significant withdrawal,” she told reporters in Geneva. “The real test for us today is if people can go and demonstrate peacefully” she added. “This is the real reality check.”

But analysts said the apparent halt in government attacks suggests Assad’s allies are pressuring him for the first time, after shielding him from international condemnation in the past. Annan has visited Russia, Iran and China to get the broadest possible backing for the plan.

On Thursday, the Russian and Chinese ambassadors called the Syrian cease-fire an important step and said they supported implementation of all points in the Annan plan – including the troop and equipment withdrawal.

“We’re encouraged that we do now have a cessation of violence in Syria,” Russia’s U.N. Ambassador Vitaly Churkin said. “We hope it holds. Everybody needs to behave with maximum prudence for that to happen.”

“Frankly, there is one thing which Mr. Annan, I hope, is going to accomplish very soon – clear-cut agreement by opposition leaders to enter into dialogue with the Syrian government,” Churkin added. “This so far has not happened.”

There were signs of how easily the Annan plan could fray.

In the hours after the 6 a.m. deadline, at least four civilians were reported killed – three of them by sniper fire – and the state-run news agency said “terrorist groups” set off a roadside bomb that killed a soldier. But there was no sign of the heavy shelling, rocket attacks and sniper fire that have become routine.

Troops also intensified searches at checkpoints, tightening controls ahead of possible large-scale protests Friday.

Although Syria promised to comply with the cease-fire, the regime carved out an important condition – that it still has a right to defend itself against the terrorists that it says are behind the rebellion.

The government denies that it is facing a popular uprising. Instead, the regime says, terrorists are carrying out a foreign conspiracy to destroy Syria. Because the regime has treated any sign of dissent as a provocation, many observers fear that an abrupt end to the bloodshed will be all but impossible.

In the early days of the Syrian rebellion, Syrian forces used tanks, snipers and machine guns on peaceful protesters, driving many of them to take up arms. Since then, the uprising has transformed into an armed insurgency, with more and more protesters taking up arms and rebels forming a fighting force to bring down the regime.

The rebel Free Syrian Army, made up largely of army defectors, has said it will observe the cease-fire. But the opposition is not well-organized, and there are growing fears of groups looking to exploit the chaos.

Saudi Arabia and Qatar have called for arming the rebels, but even if they follow through there is no guarantee that such efforts could cripple Assad’s well-armed regime.

Western powers have pinned their hopes on Annan’s plan, in part because they are running out of options. NATO-style military intervention has been all but ruled out, in part because the conflict is so explosive. Syria has had a web of allegiances to powerful forces including Lebanon’s Hezbollah and Shiite powerhouse Iran, and conflict could spark a regional conflagration.

With Thursday’s relative ease in violence, many see a U.N. observer team as a key next step.

“It is difficult to fully assess the situation on the ground, in the absence of U.N. observers,” Ban told reporters. “And therefore we are working with the Security Council to send an observer team as promptly as possible.”