They said several houses in the Masha at-Tayyar district in southern Hama were destroyed by a big explosion.
State media said 16 people died in the blast in a house used as a bomb factory by "armed terrorist groups".
The violence comes despite a UN-brokered ceasefire - part of a peace plan proposed by the joint UN-Arab League envoy Kofi Annan.
Following the blast in Hama, activists posted video on the internet showing a scene of devastation, with bodies being pulled from the rubble.
One report said 13 children and 15 women were among the dead.
They said the blast was caused by government shelling or even a Scud missile attack.
The opposition Syrian National Council has called for an emergency UN Security Council meeting "so that it can issue a resolution to protect civilians".