At least 39 people have been killed in a mortar bomb attack on an election campaign tent packed with supporters of President Bashar al-Assad, Syrian state TV has said.

The network showed pictures of Assad supporters in the southern city of Deraa dancing inside the tent. It then showed people lying dead and wounded on the ground, including children. 

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Opposition activists from the UK-based Syrian Observatory for Human Rights had earlier said that the attack on Thursday killed 21 and wounded at least 30. 

Ahmad Masalma, an opposition activist in Deraa, told the AP news agency that rebels from the Free Syrian Army fired a mortar shell at the tent in a government-held area, after repeatedly telling civilians to stay away.

He said there were about 100 people in the tent.

Thursday's attack was the first to target an election event, raising security concerns for those planning to vote.

Mohammad al-Hanus, Deraa's governor, told state television that "the terrorists' crime would not prevent the Syrians from voting", using the regime's term for its opponents.

Ban Ki-moon, the UN secretary-general, condemned the attack and reiterated his opposition to indiscriminate attacks on civilians.

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 it.

More than 160,000 people have been killed in Syria's three-year-old conflict, according to the Observatory, which monitors violence on both sides through a network of sources in the country. Millions more have fled their homes. 

Source: Agencies