Syrian army shelling and gunfire killed 34 people, including children, on Sunday in the town of Souran in the central province of Hama, the British-based Syrian Observatory for Human Rights said.
"The army shelled the town and then stormed it," the head of the group Rami Abdelrahman told Reuters, citing residents.
Hama has been a focal point of Syria's 14-month uprising against President Bashar al-Assad.
Activist video footage posted on the internet showed dead men wrapped in rugs with mourners crying around them. One showed bulldozers which activists say were used to dig mass graves.
Dozens of men stand around the vehicles, preparing the bodies for burial, a voice off camera says.
Assad's government has always been restrictive in granting access to the country for the foreign media but since the uprising started in March 2011, authorities have been especially restrictive, making it hard to verify reports.
More than 9,000 people have been killed in the government crackdown, the United Nations says. The Syrian government says 2,600 police and security members have been killed by armed groups.