More than 50 people were killed in Hawla, in Syria's Homs province, on Friday, according to the Syria Observatory for Human Rights.
The group said 13 of the dead were children, many of whose bloodied bodies were shown in a YouTube film linked to the rights group's website, dpa reported.
"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 rights group said in a statement.
The group estimated injuries at 300 or more, and anticipated more deaths.