BEIRUT -- Syrian security forces opened fire on protesters Friday killing at least six people as thousands rallied across the country to call for the downfall of President Bashar Assad's regime, activists said. Troops also clashed with armed anti-regime forces in central regions for a third straight day.

The London-based Syrian Observatory for Human Rights said six people were killed in the central cities of Hama and Homs as well as the central town of Rastan, where the clashes between troops and army defectors has been raging for days. Syria-based rights activist Mustafa Osso put the death toll at 11.

It was impossible to resolve the discrepancy or to independently verify the death toll.

The protests spread from the capital, Damascus, and its suburbs to the southern province of Daraa, the northwestern province of Idlib as well as Hama and Homs.

Many of the protesters expressed solidarity with residents of the rebellious town of Rastan just north of Homs.

Amateur videos posted online by activists showed thousands of people shouting in support of the rebellion in Rastan, where fighting continued Friday.

"Rastan will overthrow the regime," read one banner waved by protesters in the Damascus neighborhood of Qadam. Many of the protesters there covered their faces with scarves or masks to hide their identities.

The Syrian government has banned foreign journalists and placed heavy restrictions on local media coverage, making it difficult to independently verify events on the ground.

The U.N. says some 2,700 people have already died in the government crackdown against the uprising that began in mid-March.

The protests on Friday followed the week's main Muslim prayer services and were similar to demonstrations held across Syria every Friday for the past six months since the uprising against Assad erupted in the country's south.

A military official said Friday that two days of clashes between Syrian troops and anti-Assad forces in Rastan killed seven soldiers and policemen.

The official said 32 Syrian troops were also wounded in the fighting as government forces conducted a "qualitative" operation on Thursday and Friday in an effort to crush "gunmen" holed up inside the town.

The government describes its armed opponents there as "terrorist armed groups," not army defectors.

The official said the gunmen had terrorized citizens, blocked roads and set up barriers and explosives, and were responsible for the deaths of the seven troops. The comments by the unidentified official were carried by state-run news agency, SANA, on Friday.

Rastan has witnessed some of the fiercest fighting in the six-month uprising against Assad, pitting the military against hundreds of army defectors, according to activists.

The town, from which the Syrian army draws many of its Sunni Muslim recruits, has seen some of the largest numbers of defections to date. A prominent human rights activist estimated there were around 2,000 defectors fighting in Rastan and nearby Talbiseh as well as in the Jabal al-Zawiyah region in the northern Idlib province.

He spoke on condition of anonymity because of the sensitivity of the subject.

The defectors, as well as reports that once-peaceful Syrian protesters are increasingly taking up arms to fight the six-month old government crackdown, have raised concerns of the risk of civil war in Syria.

Syria has a volatile sectarian divide, making civil unrest one of the most dire scenarios. The Assad regime is dominated by the Alawite minority, an offshoot of Shiite Islam, but the country is overwhelmingly Sunni Muslim.

The report carried by SANA Friday was an acknowledgment of the stiff resistance and ongoing clashes in Rastan.

The military official said the confrontation resulted in the killing and detention of many of the gunmen. He said Syrian troops were still pursuing members of the terrorist groups in an effort to restore security to Rastan.