By Alain Iloniaina

ANTANANARIVO, March 4 (Reuters) - Madagascar's security forces fired teargas at hundreds of protesters on Wednesday during several hours of clashes in the capital Antananarivo.

Riot police deployed around the city's main square at dawn to thwart a midday rally called by opposition leader Andry Rajoelina, who has denounced President Marc Ravalomanana as a dictator and demanded he resign.

A power struggle between the two men has triggered the Indian Ocean island's worst unrest in years, killing more than 130 people and hammering its $390-million tourist sector.

"Our mission is to prevent the demonstrators from occupying the May 13 Plaza," one officer told Reuters at the square as the first volleys of teargas canisters fizzed towards the crowd.

There was no immediate word of any injuries.

Rajoelina, the capital's sacked mayor and a charismatic former disc jockey, has called for daily nationwide street protests and vowed not to return to the negotiating table for talks aimed at resolving the political crisis.

Seen by some as a maverick politician, 34-year-old Rajoelina has denounced Ravalomanana, a self-made millionaire accused by critics of monopolising business and running the world's fourth largest island like a private company.

Ravalomanana, who denies allegations of corruption, has said he will remain in power until his mandate ends in 2011.

Security forces patrolled streets around the May 13 Plaza -- the epicentre for popular revolt since Madagascar won independence from France in 1972 -- from sunrise, setting up roadblocks around the perimeter.

Some opposition supporters threw rocks and burning tyres as clouds of teargas pushed the crowds back.

"We are incensed. Why stop us from protesting and expressing ourselves in this way?" asked student Solofo Rakotonandrasa, who had been chased into the nearby city market.

U.N. Secretary-General Ban Ki-moon has named a permanent mediator to the negotiation process which continues for now without Ravalomanana and Rajoelina.

Tiebile Drame, a former foreign minister of Mali, is due in Madagascar later on Wednesday, U.N. sources told Reuters.

The Africa Union has also sent two senior officials to try to bring the two protagonists back to the negotiating table.

Recently, Madagascar has opened its doors to foreign multinationals looking to exploit its vast riches in oil, cobalt, nickel uranium and ilmenite. However, more than 70 percent of the population still live on less than $2 a day. (For factboxes on Madagascar please click [ID:nL799617] and [ID:nLR162452]) (Writing by Richard Lough; Editing by Wangui Kanina)

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