More than 70 people were killed across Syria on Friday, monitors said, as tens of thousands of protesters took to the streets and tanks were deployed in the country's second largest city.
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.
Diplomats in Geneva said the former UN secretary general would visit the Syrian capital early next week.
A report by current UN leader by Ban Ki-moon said meanwhile that 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, characterized by regular violence, deteriorating humanitarian conditions, human rights violations and continued political confrontation," said the report, obtained by AFP on Friday. It is to be debated by the UN Security Council next week.
More than 50 civilians, including 13 children, were killed in army shelling of Houla, a town in the central province of Homs, the Syrian Observatory for Human Rights said.
"It was a real massacre that took place and the UN observers are just staying silent," the head of the monitoring group, Rami Abdel Rahman, told AFP in a telephone call.
For the first time since the uprising against President Bashar al-Assad's regime erupted 14 months ago, army tanks rumbled through Aleppo, said the Britain-based watchdog.
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.
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Another four civilians were also killed by gunfire in Aleppo, the rights watchdog added, including two children.
Abdel Rahman told AFP in Beirut the protests in Aleppo were the biggest in the city since the uprising started in March 2011.
In all more than 70 people were killed on Friday, according to an Observatory tally, including 10 in Hama province, also central Syria.
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 in at least five residential neighbourhoods of the capital in support of the rebel Free Syrian Army (FSA), made up largely of dissidents 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.
Germany's foreign ministry said it had summoned the Syrian envoy to Berlin to express concern over the ongoing repression by the regime and urge immediate implementation of the UN-backed peace plan.
Annan, who brokered the six-point peace plan, is to travel to Syria "soon" as he continues efforts to find a peaceful solution to the crisis, his spokesman Ahmad Fawzi said.
"We have almost finalised the date," Fawzi told AFP, without giving further details for security reasons.
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.