Syrian forces are said to have killed 24 civilians as tens of thousands of people called on President Bashar al-Assad to step down in some of the biggest demonstrations since a three-month uprising began.

Defying Assad's military crackdown, demonstrators took to the streets again after Friday prayers across the country, from towns near the western Lebanese border to the desert regions near Iraq in the east.

'Bashar get out of our lives', read placards carried by thousands of Kurds who marched in the northeastern city of Amouda, according to a YouTube video taken by a resident.

Encouraged by the widening protests, prominent opposition figures plan to convene a 'national salvation' conference in Damascus on 16 July to reach a broad based blueprint for solving Syria's political crisis.

'In light of the military solution chosen by the regime to end the revolution, the conference aims to reach a consensus guided by the popular protest movement for a transitional period and a national salvation government that lays the foundation for a new constitution and free elections,' said a statement by the organisers.

The statement was signed by 50 figures, including Kurdish leader Mishaal al-Tammo, former judge Haitham al-Maleh, Nawaf al-Bashir, a tribal leader from the eastern province of Deir al-Zor, economist Aref Dalila, a fierce critic of the Assad's family's involvement in business and Walid al-Bunni, a physician who played a major role in a movement for democracy crushed by Assad ten years ago known as Damascus Spring.

With an intensifying security campaign that rights campaigners said resulted in arbitrary arrests of over 1,000 people over the last week alone, organisers said the conference would be far more difficult to convene than a meeting of intellectuals allowed by the authorities last week that gave a rare platform to several opposition figures.

Lawyer Razan Zaitouna said by phone that the 24 dead included seven protesters in the central city of Homs, scene of widening protests against Assad. 14 villagers were also killed in the northwestern province of Idlib, where troops backed by tanks and helicopters have been storming villages to subdue dissent.

The assaults concentrated on the northern section of Jabal al-Zawya region, home to 15,000 people, many of whom are trying to flee to Turkey, which already has 10,000 refugees from attacks in Idlib earlier this month.

In the city of Hama, video footage appeared to show tens of thousands of protesters massed in a central square. Witnesses and activists said demonstrators in Hama and in Kurdish eastern areas carried red cards, employing a soccer symbol to demand Assad's 'sending off'.

Authorities have banned most international media from operating in Syria since the outbreak of the protests in March, making it difficult to verify reports from activists and authorities.

Protesters have taken to the streets for 14 weeks to protest against Assad.

The unrest has claimed the lives of around 1,300 civilians, with security forces arresting over 12,000 people and shooting security personnel who refused to fire on civilians, according to rights groups.

Authorities say 500 police and soldiers have been killed by gunmen they also blame for most of the civilian deaths.