Tehran - President Mahmoud Ahmadinejad was scheduled to face his main challenger, Mir-Hossein Moussavi, in a televised debate Wednesday night that was expected to be watched by tens of millions of Iranians before they vote in the June 12 presidential election.

Among the three serious challengers, Moussavi has emerged as the main threat to the incumbent president, making Wednesday's direct debate one of the highlights of the electoral campaign.

A first debate, aired Tuesday on state-run network IRIB between the two other candidates, Mehdi Karroubi and Mohsen Rezaei, turned into an unspectacular question-and-answer session.

Many Iranians were hoping Wednesday that the moderators would allow Ahmadinejad and Moussavi to have an unrestrained debate.

Observers consider Ahmadinejad as the more eloquent of the two, but Moussavi has the advantage to expose the president's failures in the last four years, including his unfulfilled economic promises and pushing the country towards renewed international isolation through his uncompromising foreign policies.

Moussavi has also voiced concern that Ahmadinejad might present statistics to defend his performance in office which opposition groups consider unverifiable.

Both Ahmadinejad and Moussavi said they are optimistic of winning the election in the first round, each relying on opinion polls whose sources are ambiguous and somewhat questionable.

Most analysts believe that neither candidate could win more than 50 per cent in the first round, meaning a runoff vote scheduled for June 19 would be necessary to determine the next Iranian president.(dpa)