Protesters across Syria have demanded President Bashar al-Assad's removal after prayers marking the end of the Muslim fasting month of Ramadan.
The protesters took to the streets defying tanks and troops besieging many cities and towns, activists and residents said.
Security forces are reported to have shot dead at least four demonstrators, including a 13-year-old boy, as they streamed out of mosques in the towns of al-Hara and Inkhil in southern Deraa province.
Demonstrations broke out elsewhere across the country, especially in Damascus suburbs, the city of Homs, 165km to the north and the northwestern province of Idlib, activists and residents said.
''The people want the downfall of the president,'' shouted protesters in the Damascus suburb of Harasta, where activists said dozens of soldiers defected at the weekend after refusing to shoot at protesters.
In the nearby suburb of Saqba a crowd held their shoes up in the air - an insulting gesture in the Arab world - and chanted ''This is your level Bashar''.
According to one activists' group, troops have killed at least 551 civilians during Ramadan, the holiest period in the Islamic calendar.
Five months into the street uprising against his autocratic rule, Mr Assad, from Syria's minority Alawite sect, is facing more frequent demonstrations.
Syrian protesters are encouraged by the demise of Muammar Gaddafi's rule in Libya, with whom Mr Assad had close ties, and rising international pressure on the ruling hierarchy.
Residents and activists have also reported increasing defections among Syrian troops, drawn mostly from the Sunni majority population but dominated by Alawite officers effectively under the command of Mr Assad's younger brother Maher.
In the capital, YouTube footage showed soldiers from core units roaming the centre in big green public transport buses, their AK-47s hanging out from bus doors, to prevent protests, which broke out nonetheless in Qaboun, Kfar Souseh, Rukn al-Din and Maydan districts, activists said.
The Syrian National Human Rights Organisation, headed by exiled dissident Ammar al-Qurabi, said pro-Assad forces, including a loyalist militia known as shabbiha, had killed at least 3,100 civilians since the uprising erupted in March, including 18 people on Monday alone.
Syrian authorities blame "armed terrorist groups" for the bloodshed and say they have killed 500 soldiers and police.
European Union governments may impose sanctions on Syrian banks, as well as energy and telecommunications companies, within a week, along with a planned embargo on oil imports from the country, EU diplomats have said.
Meanwile, the Obama administration froze the US assets of Syrian Foreign Minister Walid al-Moualem and two other senior Syrian officials in response to Syria's increasingly violent crackdown against anti-government protesters.
The US Treasury Department also named Syria's Ambassador to Lebanon Ali Abdul Karim Ali and President Bashar al-Assad's adviser Bouthaina Shaaban.
“We are bringing additional pressure to bear today directly on three senior Assad regime officials who are principle defenders of the regime's activities," said David Cohen, Treasury's Under Secretary for Terrorism and Financial Intelligence, in a statement.
