BEIRUT – A Syrian peace plan brokered by the Arab League unraveled Friday as security forces killed 15 people, opening fire on thousands of protesters who denounced President Bashar Assad and said he never intended to hold up his end of the deal to end the violence.

The bloodshed, only two days after Syria agreed to the deal, suggests Damascus is unwilling — or unable — to put a swift end to a crackdown that already has killed 3,000 people since the uprising began in March.

“This regime is not serious about ending its brutal crackdown,” said Mustafa Osso, a Syria-based human rights lawyer. “Today was a real test for the intentions of the regime and the answer is clear to everyone who wants to see.”

The crisis in Syria has burned for nearly eight months despite widespread condemnation and international sanctions aimed at chipping away at the ailing economy and isolating Assad and his tight circle of relatives and advisers. The protesters have grown increasingly frustrated with the limits of their peaceful movement, and there are signs of a growing armed rebellion in some areas.

Some protesters even are calling for the kind of foreign military action that helped topple Libyan dictator Moammar Gadhafi.

But NATO has ruled out any plans for Syria, a country of 22 million with a combustible mix of sectarian and religious identities, and Assad still has a firm grip on power. The iron loyalty of his security apparatus sets the stage for an increasingly destructive fight over the future of a nation ruled for more than four decades by the Assad dynasty.

Tremors from the unrest in Syria could shake the region. Damascus’ web of allegiances extends to Lebanon’s powerful Hezbollah movement and Iran’s Shiite theocracy. And although Syria sees Israel as the enemy, the countries have held up a fragile truce for years.

Thousands of protesters braved cold and rainy weather Friday after opposition groups called for a large turnout to test whether the regime would in fact refrain from using deadly force, as agreed under the Arab League plan. But gunfire erupted shortly after the protests began, following the same pattern seen during previous Friday protests for months.

“Arab League, beware of Bashar Assad!” read one banner carried by protesters in the central city of Homs, which has turned into one of the country’s most deadly areas due to the military crackdown and what appears to be growing sectarian bloodshed.

Two main activist groups, the British-based Syrian Observatory for Human Rights and the Local Coordinating Committees, said at least 15 people were killed Friday, most of them in Homs and suburbs of the Syrian capital.

The violence was a blow to the 22-nation Arab League, which announced Wednesday that Damascus had agreed to a broad peace plan that also called for the Syrian government to pull tanks and armored vehicles out of cities, release political prisoners and allow journalists and rights groups into the country.

Officials from the Cairo-based Arab League could not be reached for comment Friday, the start of a holiday weekend.