UN officials have said Syrian security forces may have committed crimes against humanity in their crackdown on protesters, as at least eight more civilians were killed in massive protests against President Bashar al-Assad's rule.

The UN concerns were expressed in a statement late Friday by Francis Deng, special adviser to Secretary General Ban Ki-moon on the prevention of genocide, and Edward Luck, the special adviser on the responsibility to protect.

"Based on available information, the special advisers consider that the scale and gravity of the violations indicate a serious possibility that crimes against humanity may have been committed and continue to be committed in Syria," the joint statement said.

The two called for an "independent, thorough, and objective investigation" of the events in Syria.

They echoed calls by Ban to the Syrian government to allow humanitarian access to areas affected by the unrest and to facilitate the visit of the UN Human Rights Council-mandated fact-finding mission to the country.

"Without these steps, it will be very difficult to defuse existing tensions and to prevent the escalation of violence," they said.

"All actors involved in the current crisis in Syria are urged to refrain from the use of force, from acts of violence, or from incitement to violence."

The Syrian Observatory for Human Rights says 1,483 civilians have been killed in four months of protests, and thousands more have been arrested.

France and Britain also denounced the deadly crackdown on protesters, particularly in the country's third city Homs.

Paris condemned the Homs clampdown, with the foreign ministry spokesman saying on Friday that the army should protect the people rather than "sow terror."

British Foreign Secretary William Hague said he was "appalled" by the killings of civilians and the "brutal violence in Homs", saying the people have been calling for change and Assad "must listen to them."

More than 50 people have been killed since in central Homs in the past week, activists have said, accusing the regime of sowing sectarian strife among the city's Christian, Sunni Muslim and Assad's Alawite minority community.

Activists had called for Friday's demonstrations on Facebook group The Syrian Revolution 2011, a driving force behind the anti-regime protests, to show support for the flashpoint city.

The Internet group also called for a general strike on Saturday.

More than 1.2 million Syrians demonstrated in the eastern oil hub of Deir Ezzor and Hama in the north, the head of the London-based Syrian Observatory for Human Rights, Rami Abdel Rahman, told AFP in Nicosia.

"More than 1.2 million people marched: in Deir Ezzor there were more than 550,000, and in Hama more than 650,000," he said.

In Homs, more than 25,000 people gathered in Al-Ulu park in the mostly Sunni Muslim Al-Khalidiyeh neighbourhood, the Observatory said.

Security forces used violence to disperse demonstrators with deaths reported in Homs, in Syria's second city Aleppo, in Idlib near the Turkish border and around Damascus, activists said.

Rights activists said two protesters were killed Friday in Homs, three in Aleppo, another in Kfar Ruma, in Idlib province, and two in the Mleiha region of greater Damascus.

Homs has spearheaded dissent against the Assad regime since pro-democracy protests erupted on March 15. Fridays -- the weekly day of rest and key Muslim prayers -- have been a day of major demonstrations.

The army had entered Homs in May to suppress calls for the fall of the regime, and swept through the city this week, arresting "armed men" and confiscating "stockpiles of weapons," according to a pro-government daily.

Syrian television denied that more than a million people had come out to protest, saying that only "about 2,000 people took part in Friday's demonstration in Deir Ezzor."

The reports of numbers could not be independently verified.

The government blames "armed terrorist group" of attempting to sow terror in the country, and state news agency Sana said a train driver was killed and several passengers hurt when their train derailed on Saturday morning after the track was sabotaged.