(CNN) -- Syrians took to the streets in several cities Sunday in outrage over a massacre in the town of Houla that left dozens dead, many of them children.
A U.N. official said 85 people were killed Friday, 34 of them children under the age of 10.
Outrage has spread across the world. British Foreign Secretary William Hague tweeted that the charge d'affairs at the Syrian Embassy in London will be summoned to his office Monday over the killings.
Videos posted Sunday on YouTube show demonstrations in cities including Daraa, Idlib, Damascus and the Hama suburbs.
"Oh Houla, we are with you until death," protesters chanted in Daraa. A demonstration in Idlib showed a U.N. vehicle among protesters.
In the Damascus neighborhood of Midan, a protestor appeared to be injured. Video showed a man being carried out. He appeared to be bleeding from his abdomen.
And in the suburbs of Hama, demonstrators called for President Bashar al-Assad to step down.
The Syrian government vehemently denied it was behind behind a massacre, and it accused world leaders of conspiring against it. Foreign Ministry spokesman Jihad Makdissi said he addressed the media Sunday "to make a clear stance against the tsunami of lies."
"We deny that the Syrian armed forces were responsible of what took place in Houla," he said.
On state-run media, the Syrian regime said "al Qaeda-linked terrorist groups committed two horrible massacres against a number of families in the towns of al-Shumariyeh and Taldo in the countryside of Homs province." The state report showed gruesome images of children' spattered with blood.
Throughout the Syrian uprising, the government has blamed violence on terrorist groups. But opposition groups and world leaders have said Syria is engaged in a brutal crackdown on dissent.
"No words strong enough to condemn the massacre in Houla," Hague tweeted Sunday. "There must be a strong international response."
He said he will be in Moscow on Tuesday and will call on Russia to support rapid and unequivocal pressure on the Assad regime, as well as "accountability for crimes."
Russia and China have previously used their veto power in the U.N. Security Council to block a resolution that many other countries said could have helped to stop the bloodshed. Russian and Chinese officials argued that the resolution focused too heavily on regime and that they wanted a more balanced resolution.
The Arab League will meet Saturday in Doha to discuss Syria, according to a senior Arab League official. Foreign ministers are expected to attend, the official sending, speaking on condition of anonymity because he was not authorized to speak with the media.
U.N.-Arab League special envoy Kofi Annan is scheduled to visit Syria on Monday, Makdissi said.
Annan brokered a six-point peace plan two months ago and both sides agreed to it. But members of the rebel Free Syrian Army said the Annan plan is "dead," with some rebels vowing to retaliate against government forces after Friday's massacre.
"After such a long wait, a test of patience and steadfastness, the joint command of the FSA inside Syria announces that it is no longer possible to abide by the peace plan brokered by Kofi Annan, (which) the regime is taking advantage of in order to commit more massacres against our unarmed civilians," Free Syrian Army spokesman Col. Qasim Saad Eddine said in a video posted Saturday.
Since the Syrian regime and opposition members accepted the plan in March, at least 1,635 people have been killed, the opposition Local Coordination Committees of Syria said Saturday.
Rebel leaders are seeking an end to the al-Assad family's 42-year rule.
Makdissi, in his remarks Sunday, accused some U.N. countries of "openly working against Syria" and rejected the notion of an armed opposition in the country.
"There is no armed opposition in Syria. There is either an intellectual opposition, and we welcome their participation in national dialogue, or there are armed terrorist gangs that refuse the political resolution," Makdissi said.
"The Houla massacres are an integral part of the so-called intelligence war -- or the psychological warfare -- against Syria," said Jamal al-Mahmoud of the state-run Department of Political Science at Damascus University, according to the state-run Tishreen newspaper. "It is a policy carried out the enemies of Syria such as the United States, Qatar, Turkey, Saudi Arabia and France to stage acts of revenge and to create chaos instead of restoring the security and the stability that the Syrian citizen needs."
But opposition activists say in Houla, entire families were slaughtered by government forces.
"This is a clear evidence that Kofi Annan's plan is dead and a clear indication that Bashar Assad and his criminal gang do not understand anything but the language of force and violence," Eddine said. He urged the U.N. Security Council to "issue urgent and swift resolutions to save Syria, its people and the entire region by forming an international coalition mandated by the UNSC to launch airstrikes" against regime forces and their strategic points.
CNN can not independently confirm details from Syria nor the authenticity of videos. The government strictly limits access by foreign journalists.
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Lt. Bassim al-Khaled, a spokesman of the rebel Free Syrian Movement, said more bloodshed is coming. The al-Assad government is using the cease-fire and peace plan "to kill more people and is trying to crush the uprising," al-Khaled said.
"So the only language this regime is going to understand is the language of the gun," al-Khaled said. "Wait and see, we will make them pay for each drop of blood which was shed."
U.N. officials say more than 9,000 people, mostly civilians, have died and tens of thousands have been uprooted since the uprising began in March 2011. Opposition groups report a death toll of more than 11,000 people.