By Lada Evgrashina

BAKU (Reuters) - An Oscar-winning screenwriter said on Friday that he would challenge President Ilham Aliyev's decade-long rule over Azerbaijan in October elections, if the opposition names him as their candidate.

Rustam Ibragimbekov's popularity would add a dash of uncertainty to polls, which are widely expected to be won by Aliyev, 51, who succeed his father in 2003 and won nearly 90 percent of the vote five years ago.

"If I am officially named as a presidential candidate, I'll have to take this on and I am not afraid to," Ibragimbekov said by telephone from Moscow.

The screenwriter last week launched an opposition coalition, the National Council of Democratic Forces (NCDF), which brings together Aliyev's foes under one flag to challenge him in the election. He was chosen on Friday to lead the group.

He still needs to be officially registered by the coalition as their presidential candidate and his candidacy approved by the Central Election Commission.

"I can't stay apart from the explosive situation in my country, which prompted me to initiate the idea of unification of opposition forces," he told Reuters.

Azeris are proud of Ibragimbekov, who co-wrote the 1994 Academy Award-winning best foreign language film "Burnt by the Sun" with Nikita Mikhalkov. But while he will likely win protest votes, analysts said it would not be enough to topple Aliyev.

Although mainly Muslim Azerbaijan has made progress in raising living standards since gaining independence in 1991, it faces criticism over its human rights record under Aliyev and his father Heydar.

Ibragimbekov warned that his supporters could take to the streets if they believed the election was rigged.

Protests against Aliyev, who got 88.73 percent of the votes in a 2008 election that was criticized by Western monitors and boycotted by opposition groups, are usually swiftly quashed by police.

The country of 9 million people located between Iran and Russia has been courted by the West because of its role as an alternative to Russia in supplying oil and gas to Europe.

Aliyev's Yeni Azerbaijan party, which officially named him as their candidate on Friday, poured scorn on the opposition's chances in the upcoming vote.

"NCDF does not have any prospects," Eldar Ibragimov, a member of Yeni Azerbaijan Party's political council, said. "We don't believe in the ability of the opposition to unite and of any of their candidates to win the election."

The ruling party also elected the country's first lady, Mehriban Aliyeva, as its deputy head on Friday in a move some experts said may pave the way to the presidency in the future for her.

(Writing by Margarita Antidze; Editing by Alissa de Carbonnel and Pravin Char)