|A Facebook group co-ordinating the protests has urged Syrians to turn up for Friday's protests [Reuters]
Syrian security forces have continued their clampdown on the pro-democracy movement, killing four protesters and arresting two leading activists on the eve of more mass rallies.
In the eastern city of Deir al-Zour, where security forces were carrying out operations in nearly all neighbourhoods, two civilians were shot dead on Thursday night, Rami Abdel Rahman, head of the Syria Observatory for Human Rights, told the AFP news agency.
Afterwards, some 3,000 people gathered in front of the house of the new governor, Samir Othman al-Sheikh, to "demand an end to the killing," he said.
Earlier, residents were out in the streets trying to prevent security forces from carrying out arrests, he said.
To the west, in Madaya, near the capital Damascus, residents told the Reuters news agency two civilians were also killed in a security sweep on the town on Thursday.
Opposition figures arrested
In Qatana, 25km south of Damascus, security forces armed with machine guns and other weapons arrived in the town in pickup trucks overnight and carried out the arrests before searching for more protesters.
The sweep came as people took to the streets of Damascus and Qatana to protest after security forces killed 11 people on Wednesday in Kanaker, 50km southwest of the capital, said human rights activists.
Among those said to have been detained were two prominent members of a national co-ordination committee for democratic change, the London-based Observatory said.
"Security forces ... arrested two known Syrian opposition figures Adnan Wehbe and Nizar al-Samadi," it said, adding that their fate "remains unknown".
Wehbe is a leader of the Democratic Socialist Arab Union Party while Samadi is a well-known Islamic personality from Douma, a protest hub in Damascus's outer suburbs.
Around another 100 people were arrested overnight in raids on houses in Damascus, the Observatory told AFP in Nicosia by telephone.
"A demonstration was held on Wednesday night on Khaled Ben al-Walid Avenue in Damascus, bringing together many young men and women who blocked the avenue for a short time," said the Observatory.
Syria has since March 15 been shaken by an unprecedented revolt against the regime of al-Assad, which accuses "armed terrorist groups" of wanting to spread chaos in the country.
Despite the ongoing crackdown, the pro-democracy movement has called on people who have remained silent so far to turn up on Friday for yet more anti-regime demonstrations.
Security forces have arrested almost 3,000 people in the crackdown.