What next in hotly contested Iran vote?

TEHRAN (Reuters) - State media declared President Mahmoud Ahmadinejad the winner of Iran's election on Friday, but challenger Mirhossein Mousavi alleged irregularities and claimed victory for himself.

The head of the state election commission said Ahmadinejad was leading Mousavi by 69 percent to 28 percent with about 19 percent of ballot boxes counted.

Here is an outline of what this means and what happens next:

* Crucially, the election commission did not say where the votes it had counted were cast. If they were predominantly from rural areas or from poorer big city neighborhoods, the early results are likely to over-estimate Ahmadinejad's final tally.

* If this were the case Mousavi could still close the gap. The commission said more than 5 million votes had been counted. State television estimated total turnout at about 70 percent of the total electorate of 46 million, or about 32 million votes, meaning about 27 million ballots remained to be counted.

* The election commission is expected to continue to announce results throughout the night. Final results are not expected until Saturday evening. If no candidate wins 50 percent of the vote, Ahmadinejad and Mousavi will go into a June 19 runoff.

* By declaring victory at a hastily organized news conference before the election commission announced its initial results, Mousavi signaled that he would not accept defeat.

* He listed a number of alleged irregularities, including a lack of ballot papers in major cities such as Shiraz and Tabriz and said many people had not been able to vote.

* If Ahmadinejad were to win the election, a key issue to watch will be how Mousavi's supporters -- who thronged the streets of Tehran nightly in the run-up to the vote -- react. Police say they have beefed up security across the capital to prevent any unrest. All gatherings have been banned until the publication of final results. Late on Friday night, Tehran streets were quiet.

(Writing by Fredrik Dahl; editing by Dominic Evans and Samia Nakhoul)