State-run Press TV said 130 candidates are competing for 65 seats in Iran's 290-member parliament, a largely symbolic body with little control over policy but influence in domestic matters.
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Polls opened this morning in 33 constituencies, and the country's Supreme Leader, Ayatollah Seyyed Ali Khamenei, has already cast his vote, said Press TV.
Ahmadinejad's opponent secured a comfortable majority of seats of the new parliament in the first round, said AP.
Reformist candidates are mainly suppressed, so today's vote is being seen mainly as a contest between those loyal to Ahmadinejad and those loyal to Khamenei, part of a growing divide in Iran's powerful clerical establishment, said Reuters.
Although Khamenei endorsed Ahmadinejad's 2009 presidential bid -- a disputed vote that led to mass riots and accusations of fraud from now-suppressed opposition leaders Mir-hossein Mousavi and Mehdi Karoubi -- relations have since devolved into a thinly-veiled struggle for power.