The state-run SANA news agency reported on Friday that the mortar attack killed an unspecified number of people.

The pro-opposition Britain-based Syrian Observatory for Human Rights said the assault, which occurred late Thursday, targeted an "electoral tent" where Assad supporters had gathered.

It said 21 people, including 11 civilians, were killed in the attack, and more than 30 wounded.

There was no immediate claim of responsibility for the attack. Rebels fighting against the Assad regime frequently fire mortars into major cities, including Damascus, from opposition-held suburbs.

Syria's presidential election is scheduled for June 3. Assad faces two little-known challengers and is widely expected to win a third term in office, despite Syria's three-year civil war, which has killed more than 160,000 people.

Assad has not been seen campaigning since he announced his plan to seek re-election last month. He was not at the gathering on Daraa on Thursday.

Next month's vote will be held only in regime-controlled areas of Syria. The country's exiled opposition and their Western backers have called the poll a farce.

dr/tj (AP, AFP, dpa)