At least 39 people were killed and more than 300 wounded when a mortar launched by Syrian rebels struck an election campaign tent packed with supporters of President Bashar al-Assad, Syrian media reported Friday.

The shelling underscored deep fears in government strongholds that rebels will escalate attacks in an attempt to disrupt an election set for June 3.

Assad is widely expected to win a third, seven-year mandate in the vote, but the West and opposition activists have criticized it as a farce since it is taking place amid a raging civil war.

Assad has not made a public appearance in more than a month, and was not at the gathering struck by the mortar shell late Thursday 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.

Many gatherings have been held in so-called "election tents" where nationalistic songs are played and supporters mingle.

'An insult to the blood of martyrs'

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. He criticized the celebratory mood in the tents.

"They have loud music and Dabka," he said, referring to a traditional foot-stomping dance. "It's very provocative and an insult to the blood of martyrs."

He said and another activist who identified himself by his first name, Ahmad, said rebels from a faction of the Free Syrian Army umbrella group fired a single mortar shell at the tent in a government-held area, after repeatedly warning civilians to stay away. Daraa is divided into a rebel-held and a government-held sector.

He said about 100 people, including members of pro-Assad militias, officers and employees, were in the tent when it was hit in the Matar district.

The Britain-based Syrian Observatory for Human Rights, which relies on activists on the ground for its reports, said at least 21 people, including 11 civilians, were killed.

UN Secretary-General Ban Ki-moon condemned the mortar attack and reiterated his opposition to the indiscriminate use of any weapons by any party against civilians.

Daraa's Gov. Khaled al-Hannus described the attack as a "massacre" and "a crime by terrorists meant to prevent Syrians from taking part in the presidential elections."

Speaking on Syrian TV, he vowed Syrians will be undeterred and insisted "every honourable citizen" in Daraa will vote for Assad.

Assad's family has ruled Syria for more than 40 years. Though this year's vote will be the first time the family has faced challengers as opposed to a yes-or-no vote on their rule, a recently passed election law makes it impossible for those leading the revolt against Assad to compete.

Security concerns rise

Rebels trying to overthrow Assad frequently fire mortar shells into Syria's major cities, including the Damascus, from opposition-held suburbs. But Thursday's attack was the first to target an election event, raising security concerns for those planning to vote.

Assad was last seen in public on April 20 when Syrian state television broadcast images of him visiting the ancient Christian village of Maaloula, north of Damascus, after government forces recaptured the village as part of a series of major advances.

On Thursday, Syrian tanks backed by massive air power rolled into the grounds of a sprawling prison in the northern city of Aleppo, breaking a yearlong rebel siege and allowing Assad's forces to close in on a nearby rebel command centre.

In a statement Friday, the UN High Commissioner for Human Rights said "a number of prisoners and detainees, in particular 53 political detainees, whose identity is known [to the UN] are at imminent risk."