The shelling underscored deep fears in government strongholds that rebels will escalate attacks in an attempt to disrupt the balloting.
Assad is widely expected to win a third, seven-year mandate in the vote scheduled for June 3, but the West and opposition activists have criticised it as a farce since it is taking place despite a raging civil war.
The 49-year-old president himself has not made a public appearance in more than a month and was not at the gathering struck by the mortar shell late yesterday in the southern city of Daraa. But campaigning has begun in earnest, with supporters waving his pictures and Syrian flags during daily demonstrations in the capital, Damascus, the coastal city of Latakia and other government-held areas.
State TV showed pictures of Assad supporters dancing in a campaign tent in Daraa. It then showed people lying dead and wounded on the ground, including several children. Its toll was the first provided by the government for the attack, which opposition activists earlier said killed 21.
Ahmad Masalma, an opposition activist in Daraa, said six such tents, festooned with posters of Assad and Syrian flags, have been set up in the past week in the city, which holds special significance as the birthplace of the uprising against his rule in March 2011.