* Opposition says ruling coalition "cooked" vote
* Wins some seats in parliament
* Djibouti an ally in West's fight against militant Islam
By Abdourahim Arteh
DJIBOUTI, Feb 23 (Reuters) - Djibouti's ruling coalition declared victory on Saturday in the Red Sea state's parliamentary election, but the opposition rejected the vote as flawed and said it planned demonstrations to protest against the outcome.
The disputed result raises the possibility of instability in Djibouti, which hosts the United States' only military base in Africa and is an ally in the West's fight against militant Islam.
Interior Minister Hassan Darar Houffaneh said provisional results showed that President Ismail Omar Guelleh's Union for the Presidential Majority (UMP) had won 49 out of 65 seats in Friday's election.
A spokesman for the Union of National Salvation (USN) opposition alliance said the vote was rigged. He said results were announced too quickly and there were incidences of ballot-stuffing and double voting.
"It is a joke that does not match the reality of what took place in the capital and in the regions of the interior," Daher Ahmed Farah said.
"That the results were announced at 5.00 am when the polls closed at 7.00 pm is quite significant. The regime has cooked it and the Djiboutian population will take note."
The ruling coalition leaders of UMP were unreachable for comment.
Farah said opposition politicians had called for protests over the vote. There was a heavy police presence in Djibouti city in a district where protesters were likely to converge.
In power since 1999, Guelleh has effectively controlled a one-party state for his last 10 years in the former French colony, whose port is used by foreign navies patrolling busy shipping lanes in the Gulf of Aden to fight piracy.
Chaotic Somalia, a haven for al Qaeda-affiliated Islamist rebels, is among its neighbours.
Friday's election was the first contested parliamentary vote since 2003 when Guelleh's party swept all 65 seats in a poll marred by allegations of fraud.
The opposition boycotted a parliamentary election in 2008 and then refused to field a candidate in the 2011 presidential election, saying that the vote would not be free and fair.
Formed in December, the USN bloc is composed of the Republican Alliance for Development, the Djibouti Development Party and the National Democratic Party.
While the provisional results showed the opposition had eaten into the UMP's complete dominance of parliament, the USN alliance said they were not a fair reflection of actual voting.
Polls had shown that support for the opposition USN alliance surged in the run-up to the election, and the group's rallies in the capital attracted large crowds.
The opposition's main goal was to overturn what they say is Guelleh's policy to stifle dissent and the right to assembly in the tiny country of about 920,000 people.
In February 2011, galvanised by the success of Arab Spring revolts that toppled dictators in Egypt and Tunisia, anti-government demonstrators in Djibouti demanded Guelleh step down and clashed with riot police. (Editing by George Obulutsa and Rosalind Russell)