Iranians have begun to cast their votes in the country's closely-fought presidential election.

Incumbent President Mahmoud Ahmadinejad is facing a strong challenge from former PM Mir Hossein Mousavi in a campaign dominated by the economy.

The election is being followed with interest around the world for signs of a possible shift in Tehran's attitude.

If no candidate gets 50% in the first round, the two front-runners will face a run-off vote.

There has been a surge of interest recently in Iran's presidential election, with unprecedented live television debates between the candidates and rallies attended by thousands.

That enthusiasm continued into election day, with reports of long queues at polling stations.

  God willing, the best and the most deserving person will be elected as the head of the executive body for a four-year period

Ayatollah Ali Khamenei
Iranian Supreme Leader

Power of women in Iran's election
In pictures: Poll fever 
State-run Irna TV said more than five million people cast their vote in the first four hours of voting.

The country's Supreme Leader Ayatollah Ali Khamenei and both the leading candidates voted early in the day, calling on Iranians to exercise their right to choose the country's next president.

"I recommend them to just vote based on their own views and decisions," Ayatollah Khamenei said as he voted.

"God willing, the best and the most deserving person will be elected as the head of the executive body for a four-year period."

Mr Ahmadinejad thanked the people of Iran "for their goodness, for their greatness, for their selflessness, their sacrifices, and for their forgiveness".

Mr Mousavi said simply: "God willing, with the nationwide participation of the public, we will see better and more beautiful days."

Youth enthusiastic

Four candidates are contesting the election, with Mohsen Razai and Mehdi Karroubi trailing the two main contenders.

In his final TV appearance before the election, Mahmoud Ahmadinejad accused his opponents of conspiring with Israelis to falsify documents and graphs to discredit him.

 IRANIAN ELECTION
All voters over 18 allowed to vote; electorate of 42.5m people
President to serve maximum of two consecutive four-year terms (or three non-consecutive)
Election won by absolute majority
Second round held between top two if no candidate wins majority

The result will be watched closely outside Iran - in the US, Israel, and European capitals - for any hint of a possible shift in the country's attitude to the rest of the world, BBC diplomatic correspondent Jonathan Marcus says.

The timing of the election is also crucial, as the US push for a new policy of engagement with Tehran cannot really get going until the outcome of the election is clear, our correspondent adds.

The live TV debates unleashed enthusiasm among the country's young population.

BBC Iranian affairs analyst Sadeq Saba says most of them appear to be supporting the moderate candidate, Mir Hosssain Mousavi.

Varied support

President Ahmadinejad draws support mainly from the urban poor and rural areas, while his rivals have huge support among the middle classes and the educated urban population.

The BBC's Jon Leyne explains the voting process in Iran
Iranian women have also shown great interest in the election and it appears many of them will be voting for the moderate candidates who have promised them more social freedoms, our analyst says.

The votes in regions with national and religious minorities are also important, as they normally vote for reformist candidates.

Mr Mousavi is an ethnic Azeri speaker and is expected to do well in his province, as is Mahdi Karrubi in his native Lorestan province.

Iran is ruled under a system known as Velayat-e Faqih, or "Rule by the Supreme Jurist", who is currently Ayatollah Ali Khamenei.

It was adopted by an overwhelming majority in 1979 following the Islamic revolution which overthrew the autocratic Western-backed Shah.

But the constitution also stipulates that the people are the source of power and the country holds phased presidential and parliamentary elections every four years.

All candidates are vetted by the powerful conservative-controlled Guardian Council, which also has the power to veto legislation it deems inconsistent with revolutionary principles.

Source: BBC NEWS