Activists said Syrian troops shelled the central city of Homs Saturday, the Associated Press reported. That followed a Friday when a shaky cease-fire mostly held, though scattered violence that left at least six protesters dead and security forces´ refusal to withdraw to barracks cast doubt on prospects the two-day-old truce would endure.

The Britain-based Syrian Observatory for Human Rights said Saturday´s bombardment targeted two rebel-held neighborhoods, but there were no reported casualties, according to AP.

On Friday, tens of thousands of Syrians came out to protest after prayers in the first large public protests in weeks, taking advantage of the relative calm after months of heavy fighting to test President Bashar al-Assad´s commitment to the cease-fire and renew their call for his removal from power.

The hard-won truce agreement, brokered by United Nations-Arab League peace envoy Kofi Annan, took effect on Thursday morning. If it holds, the U.N. Security Council could authorize a 30-person team of observers to deploy to Syria as early as next week to monitor the truce, to be followed soon after by a beefed-up team of up to 250 observers.

A spokesman for Mr. Annan, Ahmad Fawzi, said Friday that the peace plan has been "relatively respected," the Associated Press reported.

Deep skepticism remains among opposition activists and within the international community. The Assad government has so far refused to withdraw its forces from Syrian cities and allow peaceful demonstrations, violating two points in Mr. Annan´s six-point peace plan.

The first significant clashes since the cease-fire took effect occurred early Friday between rebel fighters and loyalist forces in Idlib province, along Syria´s border with Turkey. Turkish television channels reported that gunfire could be heard in Turkish villages close to the border.

Opposition smugglers, fighters, and humanitarian workers regularly traverse the border with supplies and weapons critical to sustaining the rebel cause. If Mr. Assad hopes to quash the rebel effort he will need to plug those routes. But by operating so close to the border, he risks dragging Turkey into the fight.

On Monday, Turkey accused Syria of violating its borders after Syrian troops fired into Turkish territory, wounding at least 25 people, including two Turkish government officials.

In response, Turkey for the first time floated the prospect of involving the North Atlantic Treaty Organization if Syrian troops again violate Turkish borders. Turkey´s President Abdullah Gul said Friday that the Annan plan was a "last chance" for Damascus.

Later on Friday, violence flared elsewhere as noon prayers wound down and protesters poured into the streets in what appeared to be substantially larger numbers than in recent months, when heavy fighting kept people indoors.

Though violence levels have been significantly dialed back since the cease-fire began, the Assad regime appears intent on keeping demonstrations tamped down, while the opposition appears intent on mustering the largest demonstrations possible—making violence seemingly unavoidable.

A heavy security presence remained deployed in full force in contested cities on Friday, manning checkpoints, surrounding potential flash points such as mosques and opening fire on demonstrators in several cities, according to opposition activists.

Activists said security forces opened fire on protesters in towns outside Damascus, in the city of Hama, in the southern province of Daraa, in Aleppo, and in Idlib province, leaving six protesters dead and a seventh in critical condition.

In Homs, an opposition hotbed near Syria´s border with Lebanon which has endured some of the heaviest regime shelling of the 13-month long conflict, residents said security forces fired a handful of mortar rounds on opposition areas of the city, continued to arrest activists and deployed tanks to disperse demonstrators.

"We don´t think Assad is committed to the cease-fire," said Fares Mohammed, an opposition organizer in Zabadani, an opposition stronghold town near the capital Damascus. Mr. Mohammed said about 3,000 people took to the streets in Zabadani, but quickly dispersed when word spread that security convoys were on their way.

"There are a lot of barriers and checkpoints and tanks in the streets still," said Mr. Mohammed. "No forces have withdrawn and Zabadani remains under siege. Assad knows if he really withdraws them, if he really stops shooting us and shelling us, then every last man, woman and child will come out to the streets to protest until he´s gone."

Activists in Homs said the city continued to suffer extended electricity outages and food shortages.

Valerie Amos, the U.N.´s chief humanitarian official, said Friday at least one million Syrians were in need of urgent aid.

"It is extremely important that negotiations to enable humanitarian organizations in Syria to deliver aid remain separate from other efforts to resolve the crisis," Ms. Amos said.

Unlike past Syrian resolutions which were vetoed by Russia and China when they were put to vote in the U.N. Security Council, the proposal to send in peace monitors appears to enjoy broader support. Still, numerous potential sticking points remain.

The current U.S.-drafted resolution is heavily critical of Mr. Assad, makes little mention of the opposition´s obligations under the cease- fire and doesn´t explicitly rule out foreign support for the armed opposition—all points China and Russia are likely to oppose.

The draft resolution also calls for "appropriate further measures," which opens the door to possible sanctions, a position that both Russia and China strongly oppose. —Joe Lauria and Joe Parkinson contributed to this article. (Copyright © Dow Jones & Company, Inc.) 04/14/12)