Opposition activist Qaisar Habib, who the New York Times says is close to al-Qaida affiliated Nusra Front, defended the targeted attack on the election tent in the government-controlled part of the city as a "legitimate target for our fire" because pro-government militia are organizing President Bashar al-Assad's election campaign.
The Syrian Observatory for Human Rights reported 21 dead, including 11 civilians, six pro-government militia members, and four unidentified people.
Friday's mortar strike against appears to be the first such election-related attack by anti-Assad forces.
The presidential election is scheduled for June 3.
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