The opposition Syrian National Council has urged the UN Security Council to act urgently after claiming that regime forces "massacred" scores of civilians, including many children, in the town of Houla.
At the same time, the Syrian Observatory for Human Rights accused the Arab and international communities on Saturday of being "complicit" in the killing, saying shelling that had begun on Friday had continued into the night.
The latest flare-up of violence came as Kofi Annan, the UN-Arab League envoy to Syria who brokered a repeatedly violated ceasefire last month, finalised plans to return to Damascus.
The Britain-based Observatory said the shelling had killed more than 90 people in Houla, including 25 children.
Amateur videos posted on YouTube showed horrifying images of children lying dead on a floor. Some of their corpses badly mangled, including at least one child whose head had been partly blown away.
The Observatory accused the international community of standing "silent in the face of the massacres committed by the Syrian regime."
Earlier on Saturday, SNC spokeswoman Basma Kodmani said "more than 110 people were killed (half of whom are children) by the Syrian regime's forces" in Houla. "Some of the victims were hit by heavy artillery while others, entire families, were massacred."
A report by UN Secretary General Ban Ki-moon said groups fighting President Bashar al-Assad now control "significant" parts of some cities and there is "considerable physical destruction" across the country.
"There is a continuing crisis on the ground, characterised by regular violence, deteriorating humanitarian conditions, human rights violations and continued political confrontation," said the report, obtained by AFP on Friday.
The report is to be debated by the Security Council next week.
Annan brokered a six-point peace plan, which included a ceasefire that went into effect on April 12 but has been breeched daily since then.
He is to travel to Syria "soon" as he continues efforts to find a peaceful solution to the crisis, his spokesman Ahmad Fawzi said. Diplomats in Geneva said the former UN secretary general would visit Damascus early next week.
On Friday, for the first time since the uprising against Assad's regime erupted 14 months ago, army tanks rumbled through Aleppo, according to the Observatory.
It also said helicopter gunships went into action against rebels, strafing mountain villages in the Latakia area of northwestern Syria, near the Turkish border, wounding at least 20 people.
At least four policemen were killed in clashes with rebels in Kansebba, in the same area, the Observatory added.
Hours after massive anti-regime rallies across Aleppo, tanks deployed in the city, Syria's economic hub, rumbling through the Kalasse and Bustan al-Kasr neighbourhoods after thousands attended a funeral, it said.
Earlier the group reported that a young man was killed in Aleppo when troops fired with live rounds and tear gas on protesters in the city, where 12 massive anti-regime rallies took place on Friday.
"Long live Syria! Down with Assad!" demonstrators chanted at the funeral, said the watchdog.
Another four civilians were also killed by gunfire in Aleppo, it added, including two children.
The Observatory's Rami Abdel Rahman told AFP in Beirut the protests in Aleppo were the biggest in the city since the uprising started in March 2011.
In northwestern Idlib province, tens of thousands also marched in the rebel-held localities of Maaret al-Numan, Saraqe, Kafrnoubol, Hass and Sarge, Abdel Rahman said.
And in southern Daraa province, birthplace of the uprising against Assad, troops fired on demonstrators, several of whom were wounded in Inkhel as they emerged from mosques after weekly Friday prayers, the watchdog said.
Democracy activists had called for protests under the slogan: "Our next rendezvous, Damascus."
Damascus and Aleppo, initially spared the deadly violence that has hit Syria since last year, have been drawn into the crisis in recent weeks and have also been the scene of deadly suicide car bombings.
Protests were reported at dawn on Friday in at least five residential neighbourhoods of the capital in support of the rebel Free Syrian Army (FSA), made up largely of deserters from the regular army.
Loyalist forces fired tear gas to disperse demonstrators marching in the Midan district, the Observatory reported, saying a child was killed by sniper fire in the suburb of Irbin.
A UN panel said on Thursday that government forces were to blame for most abuses in the violence that has raged on daily despite the ceasefire supposed to take effect April 12.
More than 12,600 people have been killed in Syria in the revolt against Assad's rule, including nearly 1,500 since the UN-backed truce was to come into effect, according to Observatory figures.