Violent clashes broke out after weekly prayers on Friday between Syria's security services and demonstrators in the southern town of Daraa, epicentre of protests, a rights activist told AFP.
"Thousands of demonstrators leaving from three mosques marched to the courthouse but security forces dressed in civilian clothing fired tear gas to disperse them. Demonstrators threw stones and clashes ensued," he said by telephone.
"The situation is very tense," he added.
Syrian security forces had deployed earlier in Daraa as thousands, including people from neighbouring towns, streamed in ready to demonstrate after the traditional prayers.
"Hundreds of members of security forces in civilian clothes armed with batons and electric batons are deployed on the streets of the city," the activist said, asking not to be named for security reasons.
Daraa, around 100 kilometres (60 miles) south of Damascus, has been the focal point of anti-government protests marred by deadly violence which human rights activists blame on the security services and the government has attributed to an "armed" group.
President Bashar al-Assad, under popular pressure to introduce major political reforms and end emergency powers which give security services great leeway to crush dissent, has ordered a probe into Daraa's protest casualties.
Thousands of people on Friday also marched in towns in northern Syria, mainly in predominantly Kurdish Hassake and Qamishli, calling for an end to emergency rule and the release of prisoners, another rights activist said.
"More than two thousand people, Kurds, Arabs and Assyrians (Christians), demonstrated in Qamishli after Friday prayers," Kurdish rights activist Radif Mustafa told AFP.
"And hundreds of Kurds rallied in the streets of Hassake, Ammuda, Derek and Deirbassiye calling for the release of prisoners and the abolition of emergency law," he added.
The demonstrators chanted "God, Syria and Freedom" and "Neither Arabs, nor Kurds, national unity," according to Mustafa.
In Douma, a suburb 15 kilometres north of Damascus, residents had formed committees to verify the identity of people arriving for a rally and check that they were not carrying weapons, according to a human rights activist.
He told AFP demonstrators and authorities reached an agreement allowing protestors to rally without the intervention of security forces.
"This agreement, so far, has been respected," he said.
New York-based Human Rights Watch said at least eight people, but perhaps as many as 15, were killed last Friday in Douma when men in civilian clothes, suspected by witnesses to be security services, opened fire at protesters.
Protesters in Douma cited by HRW said men in civilian clothes, whom protesters believed to be security services because they were positioned behind riot police, opened fired with Kalashnikovs at demonstrators last Friday.
Syrian authorities again pinned responsibility on an unnamed "armed group."
Syrian rights activists estimate more than 130 people have been killed in clashes with security forces, mainly in Daraa and the port city of Latakia, since the start of political unrest on March 15.
Officials have put the death toll at closer to 30 and blamed the violence on "armed" groups and foreign interference seeking to divide the ethnically and religiously diverse country.