"The death toll from the Daraa attack has risen to 37," Rami Abdel Rahman, director of the Observatory told AFP, pointing out that among them were 19 civilians, including four children, 12 members of the popular defense committees and six soldiers.
The attack also wounded dozens, said the Observatory, which had initially reported a death toll of 21.
Syria is holding an election next month that looks all but certain to give President Bashar al-Assad a third seven-year term. His opponents have dismissed the vote as a farce.
More than 160,000 people have been killed in Syria's three-year-old conflict, according to the Observatory, which monitors violence on both sides through a network of sources in the country. Millions more have fled their homes.