(Adds second bomb, background)
TEHRAN, May 28 (Reuters) - An explosion hit a mosque in the southeast Iranian city of Zahedan on Thursday, killing 15 people and wounding 50, the state news agency IRNA reported.
Shortly after the explosion, security forces discovered and defused a second bomb near the mosque, the semi-official FARS news agency reported.
FARS said the first explosion was a suicide bombing and said it had killed 10 people and wounded more than 25.
Zahedan is the capital of Sistan-Baluchestan province, which shares a border with Pakistan and is the scene of frequent clashes between Iranian police and drug dealers and militants.
Campaigning is under way in Iran ahead of the June 12 presidential election, in which President Mahmoud Ahmadinejad is seeking a second term and faces three challengers. (Editing by Tim Pearce)