Thousands of Syrians called for the overthrow of President Bashar al-Assad on Sunday at a funeral for protesters killed by security forces in the southern town of Nawa, a witness said.
"Long live Syria. Down with Bashar!" mourners chanted, their calls audible in a telephone call during the funeral in Nawa, 25 km (15 miles) north of the city of Deraa where demonstrations against Assad's authoritarian rule first erupted last month.
"Leave, leave. The people want the overthrow of the regime."
The witness said four people were killed on Saturday in Nawa as they gathered to protest against shootings a day earlier.
At least 100 people were killed across Syria on Friday, the highest toll in five weeks of unrest, when security forces shot protesters demanding political freedoms and an end to corruption in their country, ruled for 41 years by the Assad dynasty.
Activists described Friday's killings as a turning point which exposed the hollowness of Assad's announcement that he was lifting a 48-year state of emergency and abolishing a hated state security court.
At least 12 more people were killed on Saturday at mass funerals for the slain protesters, and rights campaigners said secret police raided homes near Damascus and in the central city of Homs on Sunday, arresting activists.
Assad assumed power when his father died in 2000 after ruling Syria for 30 years. The hostile chants in Nawa on Sunday reflect a steady hardening of the demands of protesters who first called for greater freedoms but now seek his overthrow.
Despite deepening his father Hafez al-Assad's anti-Israel alliance with Iran, clawing back Syrian influence in Lebanon and backing militant groups Hezbollah and Hamas, Assad has kept Syria's front line with Israel quiet and held indirect peace talks with the Jewish state.
International condemnation of Assad has also intensified. Western criticism was initially muted because of lingering hopes that Assad might implement genuine reform and because revolution in Syria would reshape the political map in the Middle East.
"I deplore the increasing violence in Syria, and am appalled by the killing of demonstrators by Syrian security forces," British Foreign Secretary William Hague said on Sunday, advising all British nationals to leave Syria.
U.S. President Barack Obama urged Assad on Friday to stop the "outrageous use of violence to quell protests". Syrian authorities, who blame the violence on armed groups, dismissed Obama's comments.
Turkey called for "maximum self-restraint" and the continuation of reforms.
A rights campaigner said security forces shot dead at least one civilian on Sunday as they deployed in the coastal town of Jabla following a pro-democracy protest the previous night.
The weekend protests stretched from the port city of Latakia to Homs, Hama, Damascus, its suburbs and southern towns. The death toll rose to around 350, with scores of missing since the demonstrations broke out on March 18, rights campaigners said.
An eminent jurists' group said on Sunday the U.N. Security Council must investigate "mass killings" by the security forces which it said may warrant prosecution by the International Criminal Court. Human Rights Watch also called for a U.N. investigation and for international sanctions on Syria.
"Those ordering and carrying out these attacks, including those firing live rounds into crowds, must be held criminally accountable," said Wilder Tayler, secretary-general of the International Commission of Jurists.
The Syrian Observatory for Human Rights said dozens of people were arrested by security forces after the demonstrations on Friday, including nine in Idlib province, four in Syria's second city of Aleppo and five in eastern Raqqa province.
Mansour al-Ali, a prominent figure from the minority ruling Alawite sect, was arrested in Homs after he spoke out against the shooting of protesters, an activist in Homs said.
The violence in Nawa, a town of 60,000 residents in the southern Hauran Plain which was the cradle of Syria's uprising, broke out when residents tried to gather in protest on Saturday, the witness at the funeral said.
Security forces opened fire on protesters, who overran headquarters of the state and military security branches. Residents said four protesters were killed. The official SANA news agency, apparently referring to the same clashes, said seven army soldiers were killed by "criminal armed groups".
Assad has ejected most foreign media from the country, so independent reports of the violence are difficult to verify.
Demonstrators have been using the Internet to get out pictures of the violence, many of which have been explicit.
One video posted on Internet site YouTube showed a crowd marching near Abbasside square in Damascus, purportedly on Friday, chanting "the people want the overthrow of the regime", before the sound of gunfire was heard.
Demonstrators raised their hands to show that they were unarmed. The fire intensified. One youth fell, with blood spurting from his head and back. His comrades lifted him but dropped his body when the sound of bullets resumed.
In a move unthinkable in Syria just five weeks ago, two Deraa lawmakers in Syria's rubberstamp parliament resigned on Saturday to protest against the killings of protesters. (Additional reporting by Khaled Yacoub Oweis and Stephanie Nebehay in Geneva; Writing by Dominic Evans; Editing by Alison Williams)
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