Mortar fire has hit an election rally in support of President Bashar al-Assad in southern Syria, killing at least 20 people, the BBC reported, citing state media and activists.
The attack happened as Assad supporters gathered in a tent in the city of Deraa on Thursday evening. President Assad was not at the event.
At least 30 people have been injured, the UK-based Syrian Observatory for Human Rights reports.
Syrians are due to vote on 3 June in an election branded a sham by the West.
The president 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 Deraa, 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.