More than 50 people were killed in Homs province Friday, according to the Syrian Observatory for Human Rights.
The group said 13 of the dead were children. Its website linked to a video clip purportedly showing the bodies in the town of Hawla.
"The massacre has been committed by Syrian regular forces, who have been shelling the city and firing heavy machine guns throughout the day in an attempt to cripple the city before raiding it," the group said.
It expressed "severe disappointment" in the UN monitoring mission in Syria, saying it had not responded to reports about the siege of Hawla. The mission "remained silent while such a massacre" occurred, the group said.
The report followed a day of protests in Damascus and Aleppo, Syria's two largest cities, amid tight security as rebels released Lebanese Shiite pilgrims abducted earlier this week.
Earlier in the day, at least 21 people were killed, according to the Observatory: one protester was killed in Aleppo when forces fired on a demonstration; six people, including an army defector, were killed in Homs; and at least 14 people were killed by government forces in the central province of Hama, including three teenagers who were shot on their farm in the village of Chizar.
Friday's demonstrations were held under the slogan: "Our next meeting takes place is Damascus," as activists sought to intensify their activities in the Syrian capital, despite the tight grip imposed by forces loyal to President Bashar Assad.
Videos posted online showed hundreds of protesters calling for the support of the opposition Free Syrian Army (FSA), which comprises many army deserters.
In Aleppo, a group of Lebanese Shiite pilgrims - who were kidnapped on May 22 as they made their way home from a pilgrimage in Iran - were freed, Lebanese Health Minister Ali Hassan Khalil said.
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