* Opposition welcome U.N. vote barring Rajoelina address
* Security forces fire tear gas at opposition supporters
ANTANANARIVO, Sept 26 (Reuters) - Madagascar's diplomatically isolated government said on Saturday it would launch a formal objection after African nations blocked its leader from addressing the United Nations General Assembly.
Prime Minister Monja Roindefo, whose government has been rejected by regional blocs and foreign powers since coming to power after a coup in March, said he would write a letter of protest to the U.N. headquarters.
"Madagascar was invited and had the right to talk. Unfortunately, a minority of African countries objected and we were denied this right," he told pro-government Viva Radio.
The Democratic Republic of Congo, speaking at the General Assembly on Friday on behalf of the Southern African Development Community, which suspended Madagascar after the coup, asked that Andry Rajoelina be barred from addressing the gathering.
The motion was carried by a vote on the Assembly floor after chaotic scenes which led to most member states abstaining.
Political turmoil has gripped the oil- and mineral-producing island for months since Rajoelina led a campaign of violent street protests that led to the military-backed ouster of former leader Marc Ravalomanana.
The U.N. called the takeover unconstitutional and several key donors froze aid worth hundreds of millions of dollars.
Madagascar's opposition applauded the U.N. action and reiterated its view that a power-sharing deal signed in Mozambique last month remained the only way out of the crisis.
"This (Assembly vote) shows that the U.N. does not recognise Madagascar's unilaterally formed government. The message is loud and clear," Emmanuel Rakotovahiny, head of former president Albert Zafy's movement, told Reuters.
Critics say that Rajoelina, Africa's youngest leader, has reneged on populist pledges to improve civil rights, bring down the price of basic foodstuffs and improve living standards.
Riot police in body armour lobbed tear gas canisters and fired warning shots above the heads of demonstrators on Saturday to thwart an opposition rally in the city centre.
Jacky Rokoto, a self-employed businessman thrown into debt by the crisis, said Rajoelina had embarrassed Madagascar in front of the entire world.
"Rajoelina has shamed us. He meant nothing he said during his street rallies. Now he talks of democracy but he sends in the military," the opposition supporter said.
"You watch, this will explode within a month," he added.
Madagascar's opposition parties have called on Rajoelina to return to the negotiating table before an International Contact Group meeting in Antananarivo on Oct. 6. Rajoelina has not yet responded and analysts expect the crisis to drag on for months. (Editing by Tim Pearce)
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