The attack happened as Bashar al-Assad supporters gathered in a tent in the city of Daraa on Thursday evening. President Bashar al-Assad was not at the event.
At least 30 people have been injured, Syrian Observatory for Human Rights reports.
Syrians are due to vote on June 3 in an election branded a sham by the West.
President Bashar al-Assad is facing two other challengers, but he is widely expected to secure a third seven-year term in office – despite a brutal civil war now in its fourth year.
Thursday’s attack happened in the al-Matar district of Daraa, said the Observatory, which relies on reports from a network of activists on the ground.
It said the strike was carried out by an Islamist rebel brigade, and killed 11 civilians – including one child – and pro-government militiamen.
The attack “is a clear message from rebels to the regime that there is not one safe area in which to hold the election”, the Observatory’s director Rami Abdel Rahman told the AFP news agency.
This is the first time in decades that Syria is holding a multi-candidate presidential election.
Previous presidential terms have been called through a referendum with just one member of the Assad family on the ballot paper.
However, the other two candidates are not widely known and have been unable to campaign on an equal footing with the president, correspondents say.
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