Syrian security forces, undaunted by the presence of Arab League observers, have killed at least 12 protesters as hundreds of thousands demonstrated against the government of President Bashar Al-Assad, opposition activists said.

Five members of the security forces were also killed in a shooting in the city of Homs, the British-based Syrian Observatory for Human Rights said Friday.

Assad, 46, has signed up to an Arab League plan for a verifiable withdrawal of his troops and heavy weaponry from towns and cities, where they have been trying to crush protests that have raged since March.

But the presence of Arab League monitors in hotspots across Syria since Monday has, if anything, energised the protesters, while provoking scepticism in Western countries.

Demonstrators determined to show the scale of their movement to the monitors threw rocks at security forces in the Damascus suburb of Douma, where troops tear-gassed the chanting crowds.

Five people were shot dead in the city of Hama and five in the city of Deraa as crowds braved army and police, the Observatory reported, adding that at least two dozen people had been injured in Douma.

"We are determined to show them (the monitors) we exist. Whether or not there's bloodshed is not important," an activist named Abu Khaled said by phone from the northern city of Idlib.

Most foreign media are banned from Syria and witness reports and videos are hard to verify.

An opposition supporter named Manhal said thousands had tried to reach the main square of Idlib to start a sit-in but failed "because the security forces are firing a lot of tear gas and a few rounds of live fire."

"People hoped the presence of monitors will prevent fierce attacks. I believe we have partial protection, I don't think they would use live fire on us in front of the monitors."

The Observatory said security forces had shot dead two people and wounded 37 in Idlib province.

Sea of protesters

Amateur video from Idlib showed monitors in white baseball caps and yellow safety vests wading through a sea of protesters.

Some rushed at the observers, trying to shout over the thousands chanting "The people want to liberate the country!."

More than 5,000 people have been killed across Syria since March - most shot during peaceful anti-government protests but many others killed in rebel attacks and local defense actions.

Protesters flooded the streets of many towns, shouting "Peaceful, Peaceful" and "The people want you executed, Bashar!"

Some held up banners with the names of those shot dead in protests. "We will not forget your spilled blood," they read.

In parts of Hama, videos showed protesters fleeing the main streets as heavy gunfire erupted in the background. In one such segment, a few men rushed back, ducking in the crackle of gunfire, to carry away a man who had fallen limp in the street.

In the Damascus suburb of Douma, protesters bore away a man whose leg had been shredded by what they said were nail bombs.

Activists in Idlib said the army had concealed its tanks in buildings on the outskirts or in dugouts.

The Arab League mission has met with strong scepticism from the outset over its makeup, its lack of numbers - due to rise from 60 to 150 - and its reliance on government transport.

A first assessment by its Sudanese head that the situation was "reassuring" prompted disbelief in the West Wednesday, but Friday Syria's ally Russia accepted the judgement.

"Judging by the public statements made by the chief of the mission (Sudanese general Mohammed) al-Dabi, who in the first of his visits went to the city of Homs, ... the situation seems to be reassuring," Russia's Foreign Ministry said on its website.

However Friday Dabi, whom some link to war crimes in Darfur in the 1990s, said the reports of his comments were "unfounded and not true," a mission statement said. – Reuters