Al Jazeera has obtained footage allegedly showing wounded soldiers being treated by protesters in the city of Deraa
Dozens of people have reportedly been arrested in the town of Madaya outside the Syrian capital, Damascus.
Residents said tanks rolled into the mountain town early on Thursday morning and that checkpoints had been set up at all entrances into the town.
"We are terrified here and don't understand why this is happening," an eyewitness told Al Jazeera. "There were no plans for protests today and neither had any protests been held in the city in the past two days."
Madaya has seen only limited anti-government rallies since the nationwide protests began last month.
The eyewitness said he saw tanks and armoured personnel carriers surrounding the town and closing off all exits at 4am. Mobile and landline communications and electricity were cut from 4am until 9am, he said.
An activist said five people had been killed when security services raided homes at dawn.
Activists are calling for big rallies to be held on Friday, dubbed 'Azadi Friday', using the Kurdish word for freedom as a gesture of national unity.
Meanwhile, international leaders continued to exert pressure on Bashar al-Assad, Syria's president, to end the violent crackdown on anti-government protesters.
Russia, an ally of Damascus, called on authorities to punish those behind the crackdown which rights groups say has left about 500 people dead.
"We are counting on Damascus to hold an effective and transparent investigation into all the incidents leading to the deaths of people, and that the guilty parties will be brought to justice," Itar-TASS news agency quoted Alexei Sazonov, a foreign ministry spokesman, as saying.
Resistance from Russia, China and Lebanon has helped to block a European push to secure a UN Security Council statement condemning Syria's crackdown.
Assad is facing international condemnation as his leadership is using troops and tanks to put down protests, and some European powers threaten sanctions if the bloody crackdown does not ease.
Turkish prime minister Recep Tayyip Erdogan's special envoys were expected to meet Assad in Syria on Thursday to convey Turkey's worries. Turkey has close ties with Assad and is hoping to convince the Syrian leader to show restraint.
The southern city of Daraa is under military siege, with reports of food and water shortages.
A similar situation was reported in the Damascus suburb of Douma, Al Jazeera's Rula Amin, reporting from the capital, said. She said the suburb was effectively sealed off, with security personnel "in almost every corner".
Big rallies have been held in the suburb, with attempts by demonstrators to march into the centre of the capital resulting in clashes with security forces.
More than 200 members have quit Syria's ruling Baath party in the southern province of Deraa, at the epicenter of the country's uprising to protest against the violence, Mustafa Osso, a rights activist, said on Thursday.
He said another 30 resigned in the coastal city of Baniyas, adding that most of those who had quit were lower-rank members.
There were also reports of divisions within the army and a growing number of soldiers refusing to follow orders and shoot at protesters. Al Jazeera has received footage of wounded soldiers, who sources say were shot by colleagues for switching allegiance in Deraa.
Al Jazeera has not been able to verify the footage.