The Syrian army unleashed a massive attack on the city of Homs early Saturday, bombing the protest hub and killing at least 260 people in a bloody "massacre", activists and witnesses said.
The Syrian government denied involvement in the violence, blaming "armed gunmen", as television stations showed images of bodies on the ground and buildings destroyed in a city turned into a war zone.
The grim toll, if confirmed, would mark the bloodiest day of the 10-month uprising against President Bashar al-Assad's regime, with the United Nations Security Council to vote Saturday on a resolution condemning the repression.
Protesters stormed Syrian embassies in Berlin, Cairo, Kuwait and London as the Syrian National Council opposition group called on the world to act to end a brutal campaign activists say has killed at least 6,000 people since March.
"Assad forces randomly bombed residential areas in Homs, including Khaldiyeh and Qusur, which resulted in at least 260 civilians killed and hundreds of wounded, including men, women, and children," said the SNC.
The "Assad regime committed one of the most horrific massacres since the beginning of the uprising in Syria," it said in a statement sent to AFP.
The Al-Jazeera and Al-Arabiya television channels showed footage of dozens of bodies and scenes of chaos as tweets claiming to be by residents said the city "is bleeding" under heavy bombardment, one counting "366 explosions tonight so far."
Al-Jazeera said witnesses spoke of nail bombs raining down and incessant shelling, while one resident, Danny Abdul Ayem, reported "non-stop bombardment ... by tank shells and mortar bombs."
A medical student told Al-Jazeera the local hospital was struggling to cope.
"There is a lack of blood, a lack of oxygen ... there is danger in the streets," he said. "We are overwhelmed. We have opened the mosque next door" to receive wounded people, he said.
AFP was not immediately able to verify the authenticity of the videos nor of the opposition and residents' accounts because of restrictions on reporting in Syria.
"It's a real massacre," Syrian Observatory for Human Rights director Rami Abdel Rahman told AFP, calling for the "immediate intervention" of the Arab League to end the killing.
The Observatory said 138 of the fatalities were caused by mortar fire in the Al-Khalidiya district of Homs, a flashpoint of the Syrian uprising which follows others in the Arab Spring revolt that began more than a year ago.
But the Syrian government denied its army had shelled Homs and accused television stations of "inciting" violence, the official SANA news agency said.
"The civilians shown by satellite television stations are citizens who were kidnapped and killed by armed gunmen," it said. It accused the gunmen of "wanting to use that information to (pressure) the Security Council".
Hours before the UN Security Council was to vote Saturday on a resolution condemning the violent repression in Syria, Russia reiterated the text "does not suit us at all".
The SNC demanded that Russia change its position and "clearly condemn the regime and hold it responsible for the massacres, to stop the killing in Syria."
Syrians must be allowed to "democratically elect a regime that ensures freedom and dignity for all Syrians," it said.
The text to be put to the vote on Saturday highlights the UN body's support for an Arab League plan for a democratic transition while leaving out explicit references to calls for Assad to step down, a diplomat said Friday.
The Syrian rights group meanwhile called on the people "to take to the streets in the towns and villages and to rise up against the regime which is committing a real massacre right now in Homs."
In London five men were arrested Saturday after entering the Syrian embassy as around 150 protesters gathered outside, police said.
And in Kuwait authorities arrested several people when hundreds tried to storm the embassy, activists and a security official said. At least two were injured in the scramble to flee after embassy guards fired into the air, they said.
Dozens of Syrian dissidents stormed their country's embassy in Cairo, ransacking the mission and setting fire to its ground floor, and about 20 protesters broke into the Syrian embassy in Berlin Friday, officials said.
The new bloodshed follows violence across the country Friday, including Damascus, after thousands defied the government crackdown to mark the 30th anniversary of a notorious 1982 massacre in the central city of Hama that killed thousands.