"The death toll from the Daraa attack has risen to 37," said Rami Abdel Rahman, director of the Syrian Observatory for Human Rights.
"Among them were 19 civilians, including four children, 12 members of the popular defence committees (pro-regime militia) and six soldiers," Abdel Rahman told AFP.
Rebel mortar hit a tent where Assad supporters had gathered in the southern city late Thursday.
The attack also wounded dozens, said the Observatory, which had initially reported a death toll of 21.
State news agency SANA accused "armed terrorist groups" of the attack against "civilian citizens gathering in a patriotic tent in Daraa city."
The election will only be held in regime-controlled areas, and has been dismissed by the opposition and its Western backers as a "farce".