Anti-government rebels lobbed a mortar shell at a campaign rally supporting President
The attack heightened fears of escalating violence in pro-regime areas as the June 3 election approaches.
Syrian state media said the mortar struck a tent crowded with Assad supporters in the border city of Daraa, killing 39 people and leaving another 200 wounded. The agency blamed "armed terrorist groups," a term the Syrian government uses to describe opposition fighters.
"Indiscriminate use of any weapons by any party against civilians is in contravention of obligations under international humanitarian and human rights law," he said in a statement.
Mohammad Khaled Al-Hanouss, governor of Daraa, said the attack "will not bend the will of the people of Daraa who cannot wait for the coming elections so as to get rid of those thieves and mercenaries who only know how to kill and destroy."
President Assad is vying for a third seven-year term leading a country that for the last three years has been gripped in a civil war that has left more than 100,000 dead and devastated much of the country. He is expected to win by a landslide in a contest that many consider to be far from democratic.
Campaigning nonetheless has kicked into high gear with "National Tent" rallies, featuring parades, nationalistic songs, live music, and face paint, under the theme: "confirming the journey to rebuild and get past the pains."
President Assad has run under the campaign slogan "Sawa" [together], prompting a cascade of mockery by rebels who have launched a hashtag campaign on social media called "Together we will bury him."
The birthplace of the uprisings in March 2011, Daraa has witnessed a surge in attacks by regime forces scrambling to wrest control of key towns in the province, such as Nawa city, about 55 miles southwest of the capital, Damascus.
Opposition activists claim Nawa has been the target of an intense 7-day bombing campaign by air force units using barrels laden with explosives and metal detritus.
Activists also displayed leaflets they said were dropped from helicopters, questioning what rebel fighters had achieved in "destroying and sabotaging the fortifications that were built by the people and the army to confront the Zionist entity." The leaflets also called upon the rebels to lay down their arms and return to the "correct path."
"One Last Chance," the pamphlet read.