BEIRUT â€"  A fragile cease-fire brokered by the U.N. took hold in Syria on Thursday with regime forces apparently halting widespread attacks on the opposition but still defying demands by international envoy Kofi Annan to pull troops back to barracks.

If the truce holds, it would be the first time the regime has observed an internationally brokered cease-fire since Bashar Assad's regime launched a brutal crackdown 13 months ago on mass protests calling for his ouster. The opposition called for peaceful protests on Friday to test the government's commitment to the accord.

There was deep skepticism that the regime would halt its fire for long, given that Assad has broken promises in the past. Also, the regime said Wednesday, on the eve of the truce deadline, that it reserves the right to respond to any aggression, potentially a pretext for breaking the truce.

Annan's plan calls for the deployment of international observers and talks on a political transition once a truce is in place. The initiative has broad international support, including from Assad allies Russia, China and Iran, and is widely seen as the last chance for diplomacy to end the violence. The increasingly militarized uprising has been veering toward an armed insurgency.

The West and its allies doubt the sincerity of the regime's pledges to comply with the truce plan, which calls on the Syrian government to allow peaceful protests. A prolonged cease-fire could threaten the regime by encouraging large numbers of protesters to flood the streets, as they did at the start of the revolt against the four-decade rule of the Assad clan. The government met those demonstrations with a harsh crackdown, and more than 9,000 people have died since, according to the U.N.

Burhan Ghalioun, head of the opposition Syrian National Council, urged Syrians to demonstrate peacefully on Friday. Protests are common on Fridays after Muslims crowd mosques for noon prayers.

"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," he said.

Analyst Salman Shaikh, director of the Brookings Doha Center, said while the cease-fire is fragile, 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 in his attempt to get the broadest possible backing for the plan.

Annan is to brief the U.N. Security Council by video conference from Geneva on Thursday afternoon.

Both the government and the opposition have said they will abide by the truce, which began at 6 a.m. Thursday.

All the flashpoints of the uprising were reported quiet in the hours after the truce took hold. The central provinces of Hama and Homs, the northern regions of Idlib and Aleppo, the capital Damascus and its suburbs, as well as Daraa to the south and Deir el-Zour to the east were all calm, said the Britain-based Syrian Observatory for Human Rights.

However, troops, tanks and armored personnel carriers continued to patrol several opposition strongholds, including Damascus and the city of Homs, activists said.

"There have been no withdrawals from checkpoints but calm is prevailing in all areas in Syria," said Rami Abdul-Rahman, who heads the Observatory.

Another activist group, the grassroots Local Coordination Committees, said regime forces carried out arrests in the Damascus suburb of Maadamiyah shortly after reinforcements entered the area. It also reported anti-regime protests at universities in the southern city of Daraa and the eastern city of Deir el-Zour, and a protest march in the northern village of Tamanaa.

The regime ignored a Tuesday deadline for withdrawing troops from population centers, prompting renewed demands by Annan that forces return to their barracks.

Turkish Foreign Minister Ahmet Davutoglu demanded that the Syrian troops withdraw, saying that "keeping the cities under pressure is not meaningful."

Turkish Prime Minister Recep Tayyip Erdogan said Turkey could seek NATO's help in case the Syrian troops violate its borders again. Syrian forces opened fire across the Turkish border on Monday, killing two people in a Turkish refugee camp.

Assad apparently is unwilling to ease control over opposition areas for fear of widespread anti-government protests.

A major test could come on Friday. Since the outbreak of the protests in March 2011, thousands have taken to the streets every week after Friday noon prayers in the mosques.

The military crackdown over the past year succeeded in preventing protesters from recreating the fervor of Egypt's Tahrir Square, where hundreds of thousands of people camped out in a powerful show of dissent that drove longtime leader Hosni Mubarak from power.

The government denies that it is facing a popular uprising, claiming instead that terrorists are carrying out a foreign conspiracy to destroy Syria. In pledging Wednesday to observe the cease-fire, the government set a major condition, saying troops reserve the right to defend themselves if attacked.

Syrian troops have been on a major offensive since late January when they attacked rebel-held areas around the capital Damascus. During the first week of February, Assad's forces began a major campaign to retake the Baba Amr neighborhood in the city of Homs that fell in the hands of the regime in early March.

Since then, Assad's forces have been retaking major rebel-held areas including the city of Idlib as well as many towns around the country. Troops also now control much of the areas that border Turkey, Lebanon, Jordan and Iraq making it more difficult for refugees to leave the country. Despite the regime gains, rebels still held some areas, including spots in the province of Homs, Hama, Daraa.

The White House cautioned Wednesday that the Assad regime has reneged on promises to stop the violence in the past.

"What is important to remember is that we judge the Assad's regime by its actions and not by their promises, because their promises have proven so frequently in the past to be empty," White House spokesman Jay Carney told reporters in Washington.

Western powers have pinned their hopes on Annan's plan, in part because they are running out of options. The U.N. has ruled out any military intervention of the type that helped bring down Libya's Muammar Qaddafi, and several rounds of sanctions and other attempts to isolate Assad have done little to stop the bloodshed.

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.