Syria forces killed 10 people on Friday as they fired on rallies supporting army defectors opposed to a crackdown that the UN said has killed more than 3,000 people.
"Ten demonstrators were killed today," Rami Abdel Rahman of the London-based Syrian Observatory for Human Rights told AFP by telephone.
The toll included seven in Dael on the outskirts of Daraa, a flashpoint town in the south; a demonstrator in Saqba on the outskirts of Damascus, one in the Al-Qadam neighbourhood of Damascus and another on the outskirts of Aleppo.
"There were massive demonstrations in several Syrian cities despite a significant deployment of security forces," Rahman said, including a rally in Deir Ezzor, "the largest since military operations ended there in August."
Syrians also staged demonstrations in the northwestern province of Idlib, the central region of Homs, the coastal city of Latakia and Damascus, the Observatory said.
Pro-democracy activists called for nationwide demonstrations on Friday in support of "free soldiers" -- a reference to defectors -- after 36 people, including 25 soldiers, were killed in clashes across the country on Thursday.
"The free men of the army do not kill the free men of the people who are calling for their freedom," The Syrian Revolution 2011, a motor of protests, said.
The Observatory has reported mounting armed resistance to security forces. Clashes in Banash and Daraa on Thursday pitted troops against deserters, who apparently mutinied rather than obey orders to shoot civilian protesters.
Western governments have issued increasingly shrill warnings that unless the Assad regime heeds popular demands for reform, the so far peaceful protest movement risks feeling it has little alternative but to turn to violence.
"More than 100 people have been reported killed in the past 10 days alone," UN human rights chief Navi Pillay said.
At least 187 children were among civilians killed in the relentless clampdown on the protests against President Bashar a-Assad's regime that erupted in mid-March.
The heavy death toll came from "sniping from rooftops and indiscriminate use of force against peaceful protesters," noted Pillay.
"The government of Syria has manifestly failed to protect its population," Pillay said.
"The onus is on all members of the international community to take protective action in a collective and decisive manner, before the continual ruthless repression and killings drive the country into a full-blown civil war," she added.
"The international community must speak with one voice and act to protect the Syrian people," she said.
China and Russia vetoed a draft resolution UN Security Council resolution proposed by European governments, which warned of "targeted measures," but not sanctions, against the regime.
On Thursday, the European Union broadened its own sanctions against Damascus by freezing the assets of an entity that "financially supports the regime."
The statement did not name that entity, but diplomats said it was the Commercial Bank of Syria, which was already sanctioned by the United States in August over its alleged financing of Syrian missiles and unconventional weapons.
EU foreign policy chief Catherine Ashton said the decision was a "direct consequence of the appalling and brutal campaign the Syrian regime is waging against its own people."
In Beirut, the Syrian ambassador denied reports that his embassy was behind the disappearance of Syrian opposition members who have gone missing in Lebanon, calling such accusations "unfounded."
"I am puzzled by these unfounded claims that have been attributed to the police chief," Ali Abdul Karim Ali told reporters after meeting Lebanese Foreign Minister Adnan Mansour.
"Such accusations affect coordination between the two countries as concerns security issues," he added.
Earlier this week, an MP with Lebanon's Western-backed opposition said police had information implicating the Syrian embassy in the disappearance of at least four opposition figures in Lebanon.
Among them is Shebli al-Aysami, 86, a co-founder of Syria's ruling Baath party, who fled his country in 1966 over political differences. He was last seen in May in eastern Lebanon.
Ali accused some Lebanese officials and MPs of seeking to undermine his country and noted that Lebanese police in recent weeks had arrested several traffickers sending weapons to Syria.
"All these issues need to be dealt with so that they don't affect security in Lebanon or Syria," he said.