Syrian security forces killed at least eight people and wounded dozens as they cracked down on protests after Friday prayers, activists said, casting doubt on whether an Arab League plan can end months of bloodshed.

The government offered an amnesty to anyone with weapons if they reported to police within a week, "as long as they did not commit any crimes of killing," state television reported.

The gesture did not appear to be part of the Arab League plan, accepted by Syria on Wednesday, under which the army would leave turbulent cities, political prisoners would walk free and a dialogue with the opposition would begin within two weeks.

Violence has, if anything, intensified since the agreement was announced, amid reports of sectarian killings.

Troops fired on protests that erupted after Friday prayers in many towns, killing at least two people in Kanaker, south of Damascus, two in the city of Homs, where tanks were again in action, and one in Saqba, near the capital, activists said.

"Lots of people fell on the ground with bullet wounds and we are afraid some will not make it," Mohammed, a Kanaker resident, said by telephone.

Tough media curbs have made it hard to verify events in Syria since protests against President Bashar al-Assad began in March, inspired by revolts against Arab autocrats elsewhere.

Syrian state television denied any killings, and aired footage it said were from areas where protests were reported, showing crowds calmly leaving mosques after prayers.

But YouTube footage, purportedly from many towns and cities, showed thousands of people waving flags, with some shouting: "Mother do not cry, Bashar's days are numbered."

One clip, from the town of Taybet al-Imam, near Hama, showed crowds marching along a main street where huge Syrian flags from the pre-Baathist era were draped over buildings, along with the Libyan flag adopted by those who overthrew Muammar Gaddafi.

Homs has emerged as a protest flashpoint and a centre for emerging armed resistance to government forces. Activists said tank and sniper fire killed at least 22 people in the central city on Thursday, mainly in the old Bab Amro quarter.

The violence in Homs, where tanks were bombarding for the second straight day, illustrates how difficult it will be to implement the Arab League plan in a country locked in a deadly struggle between Assad and foes of his 11-year rule.

Diplomats who have seen the agreement say it lacks any timeline for implementation.

Fears that unrest is taking a sectarian turn have mounted this week amid reports of killings of members of Assad's minority Alawite community and counter-killings of Sunni Muslims, who form a majority of Syria's 20 million people.

State news agency SANA quoted several Homs residents describing attacks by gunmen on shared taxis on Thursday.

One woman, named Ikhlas Ashour, said gunmen forced passengers out of the taxi and robbed them of their money and identity cards before killing all the men.

"It was an indescribably ugly scene, I felt like I was living a nightmare," she was quoted as saying.

Another resident, Ghayath Darwish, told SANA gunmen killed some male passengers at random and dragged away their corpses.

SANA also said the bodies of 13 soldiers killed by "armed gangs" in Homs, the city of Hama and the northwestern province of Idlib were returned to their families for burial on Thursday.

The opposition has so far rejected talks with Assad as long as violence continues and has said the only way to restore peace is for the president to step down immediately.

"We have already seen the regime's bloody response to the Arab initiative today in the form of intensified shelling on Homs," Ahmad Ramadan, spokesman for the opposition Syrian National Council, said on Thursday.

Security forces arrested dozens of people in the early hours in some northern Damascus suburbs, activists said.

In Maarat al-Numaan, a town on the Damascus-Aleppo highway, one resident said troops manned roadblocks and snipers lurked on rooftops. Nevertheless, crowds were shouting "Freedom, freedom, despite you, Assad," he said.

The United Nations says more than 3,000 people have been killed since the uprising against 41 years of Assad family rule began in March. The authorities accuse Islamist militants and foreign-backed armed gangs of killing 1,100 security forces.

Western sanctions and growing criticism from Turkey and Arab neighbours have raised pressure on Syria to end the bloodshed.

The United States said on Thursday it saw no evidence that Syria was taking steps to fulfil the Arab League deal.

"This Assad regime has a long deep and continued history of broken promises and it has significant blood on its hands," State Department spokeswoman Victoria Nuland said.