Security forces and gunmen loyal to President Bashar Assad stormed the large Damascus suburb of Douma early on Monday, shooting at unarmed civilians and arresting residents, rights campaigners said.
"There are injured people. Scores have been arrested. The security are repeating the same pattern in all the centers of the democratic uprising. They want to put down the revolution using the utmost brutality," the rights campaigner told Reuters from Damascus.
Residents in the southern town of Deraa, where the protest movement first erupted last month, said security forces backed by armoured vehicles had entered the town overnight and were shooting.
An eyewitness told The Associated Press that he saw at least five corpses after security forces fired on a car. The witness asked that his name not be used for fear of reprisals.
Also Monday, a senior diplomat in Amman said Syria had closed all its land border crossings with Jordan, following the deployment of Syrian army tanks in Deraa. An official told Reuters the "timing is related to what appears to be a major security operation that is taking place right now".
Rights groups say security forces have killed more than 350 civilians since unrest began last month. A third of the victims were killed in the past three days as the scale and breadth of a popular revolt against President Bashar Assad intensified.
Foreign journalists have mostly been expelled from the country, making it impossible to verify the situation on the ground. Grisly footage posted on the Internet by demonstrators in recent days appears to show troops firing on unarmed crowds.
In some of the latest violence, activists said government troops and gunmen loyal to Assad shot dead at least nine civilians on Sunday in the Mediterranean coastal town of Jabla, where troops deployed following a protest the previous night.
Rights campaigners said they feared Assad's forces were preparing for an attack on the town of Nawa after reports of bulldozers and military vehicles heading there. Thousands of people in the town called for the overthrow of Assad on Sunday at a funeral for protesters killed by security forces.
Electricity and communications were cut off in parts of Nawa by the evening and residents, some armed, erected barriers in the streets in preparation to defend against attack.
"Long live Syria. Down with Bashar!" mourners chanted during the funeral in Nawa, 25 km (15 miles) north of Deraa.
"Leave, leave! The people want the overthrow of the regime."
In Banias, south of Jabla, protest leaders said they would cut the main coastal highway unless the siege on Jabla was lifted. Jabla is home to large numbers of members of Assad's Alawite Shi'ite minority who had generally stayed away from protests in the past.
Mourning 'martyrs of uprising'
Monday's declaration was signed by 102 writers and journalists, in Syria and in exile, representing all the country's main sects, a sign that shock at the violence is crossing Syria's lines of sectarian division.
It called on Syrian intellectuals "who have not broken the barrier of fear to make a clear stand.
"We condemn the violent, oppressive practices of the Syrian regime against the protesters and mourn the martyrs of the uprising."
Signatories included Alawite figures such as former political prisoner Loay Hussein; female writers Samar Yazbek and Hala Mohammad; Souad Jarrous, correspondent for al-Sharq al-Awsat pan-Arab daily; writer and former political prisoner Yassin al-Haj Saleh and filmmaker Mohammad Ali al-Attassi.
Mansour al-Ali, a prominent Alawite figure from the city of Homs, was arrested in his home city after he spoke out against the shooting of protesters, an activist in Homs said.
At least 100 people were killed across Syria on Friday, the highest toll of the unrest, when security forces shot protesters demanding political freedoms and an end to corruption in their country, ruled for 41 years by the Assad dynasty.
Another 12 people were killed on Saturday at mass funerals for slain protesters. Rights campaigners said secret police raided homes near Damascus and in the central city of Homs on Sunday, arresting activists.
AP contributed to this report
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