Syrian security forces have arrested at least 500 pro-democracy activists, a rights group said, as the government continues a violent crackdown on anti-government protests across the country.
The arrests followed the deployment of Syrian troops backed by tanks and heavy armour on the streets of two southern towns, the Syrian rights organisation Sawasiah said on Tuesday.
The group said it had received reports that at least 20 people were killed in the city of Deraa in the aftermath of the raid by troops loyal to Syrian president Bashar al-Assad on Monday. But communications have been cut in the city, making it difficult to confirm the information.
"Witnesses managed to tell us that at least 20 civilians have been killed in Deraa, but we do not have their names and we cannot verify," a Sawasiah official told the Reuters news agency.
The group said that two more civilians were confirmed dead after government forces entered Douma, a suburb of the capital, Damascus.
At least 500 people were arrested elsewhere in the country, it said.
Deaths and arrests
Gunfire continued to reverberate across the city of Deraa on Tuesday, residents said, a day after thousands of soldiers swept into the city, with tanks taking up positions in the town centre and snipers deploying on rooftops, witnesses said.
"We've been listening to live ammunition. Some snipers are working as well, but we don't know from where," a resident of Deraa told Al Jazeera on Tuesday.
"The snipers are on all the roofs. I'm now on my stomach, on the ground - I am really in a panicked situation. The city is quite in danger."
Witnesses said soldiers began opening fire on civilians indiscriminately after arriving in Deraa, sparking panic in the streets.
However, the government insists the army was invited in to rid the town of gunmen.
Al Jazeera's Rula Amin, reporting from Damascus, said the government has reiterated that it is there to protect residents.
A Deraa resident describes to Al Jazeera a desperate situation on the ground in the restive southern city
"What we are hearing from activists in Damascus is fear and concern that what the government is trying to do is crush the protests to create fear among people to stay at home.
"Then [the government will] come up with its own plan of reforms, but people won't be able to stand up and defy these reforms. That is how the government wants to move forward."
She said the troop deployment was an "unprecedented" offensive against the wave of dissent that has swept the country since the uprising began on March 15.
Up until now, she said, security forces had cracked down in reaction to protests. But the flood of troops into Douma and Deraa came in the absence of any demonstrations.
"We're seeing a different tactic, with security forces sweeping the towns," she said, noting reports of house-to-house searches, arrests and random shooting coming from both towns.
Also for the first time, the military has become directly involved in quelling the uprising, much to the disappointment of opposition activists.
"They were hoping the army would not get involved," our correspondent said. "They feel this is only the beginning of a very serious crackdown."
However, one activist told Al Jazeera that some army officers have defected to fight alongside the people of Deraa against the government.
Two members stepped down from the provincial council in Deraa. The resignations came a day after two legislators and a religious leader from Deraa broke with the government in disgust over the killings.
Protesters gunned down
Meanwhile, in the coastal town of Jableh, where several protesters were gunned down on Sunday, witnesses said security forces in camouflage uniforms - some with their faces covered - and masked armed men dressed in black were roaming the town's streets.
The Syrian Observatory for Human Rights, a Syrian rights group, said on Monday that at least 13 people had been killed in Jableh since Sunday's crackdown began.
The country has banned nearly all foreign media and restricted access to trouble spots since the uprising began, making it nearly impossible to get independent assessments.
Syria has also closed all border crossings on its southern frontier with Jordan as the crackdown intensifies, a security official told Al Jazeera.
Syrian intellectuals have expressed their outrage over the violence, with a declaration on Monday signed by 102 writers and exiles from all the country's main sects.
"We condemn the violent, oppressive practices of the Syrian regime against the protesters and mourn the martyrs of the uprising," they said.
President Assad is also coming under increased foreign pressure to stop the deadly crackdown.
France, Britain, Germany and Portugal have all urged the UN Security Council to condemn the government's violent action against demonstrators, and the United States is considering imposing new sanctions.