Heavy shooting has been reported in a suburb of the Syrian capital, Damascus, after the army cordoned off the area.
Syrian security forces were also carrying out house-to-house raids across the country, detaining hundreds of people as part of a widening crackdown on anti-government protests, rights activists said.
Rami Abdul-Rahman, director of the Syrian Observatory for Human Rights, said the arrests were targeting the demonstrations’ organisers and participants.
He said today’s were focused on the central city of Homs, the coastal city of Banias, some suburbs of the capital Damascus and villages around the southern flashpoint city of Daraa.
President Bashar Assad’s regime also sent tanks deep into Syria's third city Homs, escalating a military campaign in an attempt to crush a seven-week-old uprising against his autocratic rule.
Syrians demanding political freedom and an end to corruption have held weeks of what they say are peaceful demonstrations in the face of government repression, despite a civilian death toll that has reached 800, according to the Syrian human rights organisation Sawasiah.
Yesterday, Homs residents said they heard machine-gun fire and shelling as troops made their first incursion into residential areas of the city of one million people, 165 km north of Damascus.
At least one person, a 12-year-old child, was killed when tanks and troops charged into the Bab Sebaa, Bab Amro and Tal al-Sour districts of Homs overnight, according to the Syrian Observatory for Human Rights.
"The areas have been under total siege since yesterday. There is a total blackout on the numbers of dead and injured, Telecommunications and electricity are repeatedly being cut in those districts," the Observatory said in a statement.
Elsewhere, a witness said security forces killed at least two unarmed demonstrators on Sunday when they fired on a night rally in the eastern city of Deir al-Zor.
Assad, who has maintained the autocratic political system inherited from his father, began by making vague promises of reforms, but when that failed to stop the protests, he made clear he will not tolerate dissent or risk losing the tight control his family has had over Syria for the past 41 years.
The pro-democracy upheaval that began in Deraa on March 18, inspired by similar revolts across the Arab world, intensified on Friday across Hauran, an agricultural belt bordering Jordan to the south and the Israeli-occupied Golan Heights to the west.
In the south, tanks swept into several towns yesterday. A man was killed when security forces smashed their way into his home in the southern town of Tafas, a rights campaigner in the region said.
Reuters