* Opposition supporters disbursed

* Analysts says turmoil to last for months

By Alain Iloniaina

ANTANANARIVO, Sept 11 (Reuters) - Security forces in Madgascar fired tear gas on Friday to try and disperse hundreds of opposition supporters gathering for a rally in the capital of the Indian Ocean island.

Backers of ousted President Marc Ravalomanana massed in a park near a central square, but security forces moved in saying the demonstration had not been authorised.

"The should have given us permission to meet at this so-called 'Democracy Square'," said protester Andry Rakotomanga.

Andry Rajoelina, the former mayor of Antananarivo, spearheaded weeks of violent street protests before toppling Ravalomanana in a March coup with the help dissident soldiers.

Ostracised by most of the international community, Rajoelina joined crisis talks last month with Ravalomanana and two other former presidents in Maputo, Mozambique.

The island's four political heavyweights signed a deal there laying out the structure of a power-sharing government, but they failed to agree on who should fill the top posts.

Rajoelina then unilaterally formed a government this week designed to steer the world's fourth largest island to presidential polls, but it has been rejected by the opposition, Southern African leaders and the African Union. [ID:nLA288142]

Analysts say turmoil is likely to rock the island for months to come, stunting investment, prolonging an aid freeze and driving economic growth into negative territory. [ID:nLB542151]

Under Ravalomanana, who was accused of abuse of office for private gain, Madagascar enjoyed sustained economic growth as the country opened its doors to foreign investors hoping to exploit its oil, nickel, cobalt, gold, uranium and coal.

Analysts say key factors determining how long the turmoil persists will be whether Rajoelina offers sweeteners to the opposition, or whether divergent opposition groups mount a united front and launch a sustained campaign of street protests. (Writing by David Clarke; Editing by Daniel Wallis)

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