Syrian forces have killed nearly 142 people, including at least 100 when the army stormed the flashpoint protest city of Hama to crush dissent on the eve of Ramadan, political activists say.
Rights groups said Thursday was one of deadliest days in Syria since demonstrators first took to the streets on March 15, demanding democratic reforms and the downfall of the government.
As reports of the brutal crackdown on Hama unfurled, Britain, Germany, France, Italy and Turkey condemned the violence, while a US diplomat said it was "full-on warfare".
"It is one of the deadliest days" since the protests erupted, Rami Abdel Rahman, the head of the Britain-based Syrian Observatory for Human Rights, said.
Death tolls provided by the observatory and other human rights groups showed at least 142 people were killed across Syria, most of them falling in Hama.
"One hundred civilians were killed on Sunday in Hama by gunfire from security forces who accompanied the army as it stormed the city," Abdel Karim Rihawi, head of the Syrian League for the Defence of Human Rights, said.
Rihawi said five other people were killed in the central city of Homs, and three more in the northwestern province of Idlib, when security forces opened fire on protesters who rallied in support of Hama.
"The number of those wounded is huge and hospitals cannot cope, particularly because we lack the adequate equipment," Abdel Rahman quoted a Hama hospital source as saying.
He said the crackdown on Hama came after more than 500,000 people rallied in the city on Friday following Muslim prayers during which a cleric told the congregation "the regime must go".
Activists also reported deaths in Deir ez-Zor, Syria's main gas- and oil-production hub in the east, which has become a rallying point for protests along with Hama.
At least 19 people were killed in Deir ez-Zor, six in Herak in the south, and one in Al-Bukamal in the east, said Qorabi, adding most of those shot in Deir ez-Zor were "hit in the head and the neck" by snipers.
The Syrian Revolution 2011, an internet group that has been a driving force behind the protests, urged demonstrators to gather nationwide after Ramadan "taraweeh" evening prayers later on Sunday "for retaliation protests". "Syria is bleeding" it said.
Western powers condemned the violence amid warnings from Berlin and Paris of fresh sanctions against the government of President Bashar al-Assad.
JJ Harder, the press attaché of the US embassy in Damascus, speaking to Al Jazeera
US President Barack Obama said Sunday he was "appalled by the Syrian government's use of violence and brutality against its own people".
In a statement, Obama saluted demonstrators who have taken to the streets as "courageous" and said Syria "will be a better place when a democratic transition goes forward".
Earlier, JJ Harder, a US diplomat in Damascus, told Al Jazeera that the violence in Hama amounted to "full-on warfare" and a final act of desperation.
"It is resorting to some desperate last ditch attempt, trying to save itself. It is full-on warfare on its own people," Harder, the press attaché of the US embassy in the Syrian capital, said.
The Syrian authorities have consistently accused "armed gangs" and fundamentalist Salafist Muslims of stirring the unrest and aiming to sow chaos in the Sunni-majority country.
Asked if he accepted the Syrian government's contention that its forces were up against armed gangs, Harder said: "The Syrian government is completely delusional. They are making up fanciful stories that no one believes."
Other Western powers joined in the condemnation of the latest killings of protesters.
Germany threatened to new sanctions on Damascus along with its EU partners, France warned Syria's leaders "will have to answer for their deeds," and Italy called the Hama crackdown "the latest horrible act of violent repression".
Criticism also came from neighbour Turkey, which said it was "deeply saddened and disappointed ... by the current developments on the eve of holy month of Ramadan".
Residents in Hama said the army entered the city with tanks early on Sunday before gunfire erupted, in an apparent operation to wrest back control after security forces withdrew almost two months ago.
The official SANA news agency said that gunmen shot dead two security forces in Hama while a colonel and two soldiers were "martyred" in Deir ez-Zor.
SANA said the gunmen torched police stations and attacked private and public property in Hama, adding that soldiers tore down barricades and checkpoints set up by the armed men at the city's entrance.
Abdel Rahman said the army also launched an operation against Muadhamiya in the Damascus region at dawn, "with tanks blocking the southern, eastern and western entrances to the town".
In-depth coverage of escalating violence across Syria
The Syrian League for the Defence of Human Rights reported more than 300 people detained in Muadhamiya.
Also arrested was Bagara tribal chief and opposition figure Nawwaf Ragheb al-Bashir, who was seized on Saturday in Deir ez-Zor, according to the Syrian League.
The observatory reported demonstrations in the central city of Homs and along the Aleppo-Damascus highway - which residents cut off in several points - to protest against the Hama crackdown.
In 1982, an estimated 20,000 people were killed in Hama when the army put down an Islamist revolt against the rule of Assad's late father, Hafez.
Earlier this month, the president replaced the governor of Hama after a record 500,000 protesters rallied in the opposition bastion on July 1 calling for the fall of the regime.
At least 1,583 civilians and 369 members of the army and security forces have been killed since mid-March in Syria, according to the observatory.