BEIRUT — A mortar shell struck a large tent in southern Syria where supporters of President Bashar Assad had gathered for election campaigning, killing at least 21 people and wounding many others.

Assad, rarely seen in public since the start of Syria’s 3-year-old conflict, has not been seen campaigning since he declared his candidacy last month and he was not at the gathering in the southern city of Daraa, where the uprising against his rule began in March 2011.

More than 160,000 people have been killed in the fighting as the revolt morphed into a civil war that has also sent millions fleeing for their lives and turned once-prosperous cities into rubble-strewn war zones.

There was no immediate claim of responsibility for the attack in Daraa. Rebels trying to overthrow Assad frequently fire mortar shells into Syria’s major cities, including the capital, Damascus, from opposition-held suburbs.

The overnight attack underscored concerns that rebels will step up attacks against government strongholds in the run-up to the June 3 election to disrupt the voting. Western leaders and opposition groups have described the vote held amid the civil war as a sham.

Ahmad Masalma, an opposition activist in Daraa, said rebels from the Free Syrian Army umbrella group fired a mortar shell at the tent in a government-held area.