Two leading Syrian opposition parties have agreed on a road map to democracy should a popular uprising succeed in toppling President Bashar al-Assad, according to a copy of the document seen by Reuters.

Hundreds of thousands had taken to the streets across Syria on Friday, aiming to demonstrate the strength of their movement to Arab League monitors checking whether Assad is implementing a pledge to halt a violent crackdown on unrest that has been raging since March.

The observer mission has already stirred controversy for its lack of numbers and comments by its Sudanese leader, General Mohammed al-Dabi, suggesting he was reassured by first impressions of Homs, one of the main centres of unrest.

He later backtracked from the reports of those remarks, but appeared likely to cause fresh concern that the mission would be soft on Syria by undermining the comments of one of his observers in Deraa, cradle of the uprising, posted in a video on YouTube early on Saturday.

"We saw snipers in the town, we saw them with our own eyes," the observer says in Arabic, visibly concerned. "We're going to ask the government to remove them immediately. We'll be in touch with the Arab League back in Cairo."

Dabi later told the BBC: "This man said that IF he sees, by his eyes, those snipers, he will report them immediately and he will tell the government. But he didn't see, he said that 'IF he (sees)', so it is not correct in the media, what he said."

The British-based Syrian Observatory for Human Rights said security forces had shot dead 27 people on Friday in areas where there were no observers, adding to the toll of a conflict that the United Nations says has killed more than 5,000 people, most of them unarmed civilians.

The Observatory said four civilians had been shot dead on Saturday, three by snipers. The bodies of three detainees were also returned home, and a woman died of gunshot wounds, it said.

With little confidence in the Arab observer mission, opposition groups are trying to create a coherent movement to build political pressure and to boost their credibility in the eyes of other countries that fear chaos if Assad is forced out.

The leading opposition group in exile, the Syrian National Council, signed the deal on Friday with the largely Syrian-based National Coordination Committee, according to Moulhem Droubi, a top SNC member from Syria's Muslim Brotherhood.

The two groups have received attention from Western powers, but it is not clear how much sway they hold with the mass of protesters. The document seen by Reuters says the deal will be presented to other opposition groups at a conference next month.

The National Coordination Committee had disagreed with the SNC's calls for foreign intervention - one of several disputes that had prevented opposition groups agreeing on what a post-Assad Syria should look like.

In their pact, the two sides "reject any military intervention that harms the sovereignty or stability of the country, though Arab intervention is not considered foreign".

The groups outlined a one-year transitional period, which could be renewed once if necessary. In that period, Syria would adopt a new constitution "that ensures a parliamentary system for a democratic, pluralistic civil state".

The document also stresses that religious freedom will be guaranteed by the new constitution and condemns any signs of sectarianism or "sectarian militarisation".

Most of the protesters come from Syria's Sunni Muslim majority, while Assad still appears to enjoy significant support among members of his Shi'ite Alawite sect, from which most of the ruling establishment is drawn.

The Arab League plan calls for a verifiable withdrawal of troops and heavy weaponry from towns and cities.

But activists say they have little faith that the Arab League mission can help to stem the violence against them.

The mission is still short of its planned strength of 150 members, who must observe events in dozens of towns and cities across a country of 23 million people. And it relies for its transport on state security escorts who some protesters say have prevented access to the demonstrators.

"We don't know what to do. But we know Assad and his regime won't give us what we want," said opposition activist Ziad in Douma, a suburb of Damascus that has seen big protests. "So why should we wait for them to help us?

"Assad wants us to raise our weapons and kill each other and he is pushing us towards that every day. We wanted the monitors to help us find a solution, but it won't happen."

SNC head Burhan Ghalioun said on Friday that if the government did not implement the peace plan, "there is no other solution except going to the (UN) Security Council - and I think we are walking toward the Security Council".

On Saturday, thousands took to the streets in the protest hotspot of Idlib, carrying the bodies of three slain protesters wrapped in white sheets and sprinkled with leaves.

"The martyr is beloved by God and Assad is the enemy of God," the protesters shouted, according to witnesses. Most foreign media are banned from Syria, making witness reports hard to verify.

Assad, 46, says Islamist militants steered from abroad are the source of unrest and have killed 2,000 of his forces.

The state news agency SANA reported at length on "massive demonstrations" throughout Syria on Friday in support of Assad, and against "the plot which Syria is exposed to".

It said demonstrators had denounced "the pressure and biased campaigns targeting Syria's security and stability" and the "lies and fabrications of the misleading media channels".

Some protesters have decided their best hope lies with the Free Syrian Army, a group of army defectors and armed rebels who have been taking the fight to Assad's forces and sometimes overshadowing the peaceful protests.

"I think it's obvious at this point that the Arab League needs to take a stronger stance. We need support for the Free Syrian Army," activist Manhal Abu Bakr said by phone from Hama.

"It has been nearly a week and they (the monitors) haven't stopped the killing."

Arab League Secretary-General Nabil Elaraby had said it should take only a week to see if Assad was keeping his word.

The commander of the Free Syrian Army told Reuters on Friday he had ordered his fighters to stop attacks while the FSA tried to arrange a meeting with the monitors.

But in a newspaper interview published on Saturday he said if the Arab mission was "not professional", the FSA would "resume our defence operations".

UN spokesman Martin Nesirky said on Friday that the United Nations was ready to train the observers in rights monitoring.