A man decorated with pictures of Iranian President and candidate for the upcoming presidential election Mahmoud Ahmadinejad attends Ahmadinejad's address to supporters in front of the Sharif University in Tehran June 10, 2009. [Agencies]

TEHRAN, Iran -- Iranians have begun voting in a key presidential election pitting hard-liner Mahmoud Ahmadinejad against a pro-reform challenger more open to improving ties with the US.

State media announced early Friday that more than 45,000 polling stations across the country had opened.

Voter turnout is expected to be high during the 10 hours of voting.

The election is seen as a referendum on Ahmadinejad. His top opponent, Mir Hossein Mousavi, has energized many youth who want fewer hard-line Islamic restrictions and better international relations.

There are about 46 million eligible voters in Iran, a country of 70 million people.