* Opposition leader assumes presidency on island
* South Africa denounces "unconstitutional" move
* New president prioritises anti-poverty fight
(Recasts, adds details, analyst, SADC)
By Richard Lough
ANTANANARIVO, March 18 Madagascar's youthful opposition leader Andry Rajoelina sought to consolidate his grip on power on Wednesday after the military named him president in a move that flouted the Indian Ocean island's constitution.
President Marc Ravalomanana resigned on Tuesday, but analysts said he had effectively been given no option after the security forces backed his foe, a baby-faced former disc jockey who has led weeks of anti-government strikes and protests.
The nation's worst unrest in years killed at least 135 people, devastated a $390 million-a-year tourism sector and worried multinationals in its mining and oil industries.
The outcome was a slap in the face for the African Union (AU), which has censured recent violent transfers of power that damage the continent's reputation with investors.
Experts said donors may cut aid to the world's fourth largest island, but probably only in the short-term.
"With so many people below the poverty line I can't see the international community abandoning Madagascar in the long run, and (Rajoelina) knows this," Lydie Boka, of Paris-based risk group StrategieCo, told Reuters.
While military support was the crucial factor that put the opposition leader in the presidency, analysts say the sacked mayor of the capital also has the backing of exiled former president Didier Ratsiraka and his allies.
Some analysts said former colonial ruler France also gave him tacit support.
Rajoelina was feted by locals as he drove through the streets on Tuesday. The new president was seeking to re-open ministries in the capital Antananarivo on Wednesday that have stayed closed and barricaded during the crisis.
Ravalomanana's whereabouts were unclear, while Rajoelina supporters planned a big party in the city's May 13 square. They had accused Ravalomanana of losing touch with the majority of the population who eke out a living on less than $2 a day.
Rajoelina says his priority will be to address social needs in Madagascar, which lies off Africa's east coast and won independence from France in 1960.
Under Malagasy law, the head of parliament's upper house should have taken over after the president's resignation and organised an election within two months.
Instead, Rajoelina -- who at 34 is six years too young to be president under the constitution -- now heads a transitional government which has pledged to hold a poll within two years.
The AU had demanded the constitution be respected "scrupulously". But the fact the army refused to take over on Tuesday, as Ravalomanana had requested, means the AU may not brand the events a coup which would have meant suspension.
After recent coups in Mauritania and Guinea as well as the killing of Guinea-Bissau's leader, Ravalomanana's fall raises doubts over the durability of democracies elsewhere in Africa.
That could further damage investor sentiment towards the continent, which had improved in the last few years partly as a result of a decrease in the frequency of coups and a perception that democracy was becoming better entrenched.
South African President Kgalema Motlanthe, who is chairman of the SADC regional trade bloc, denounced the change of power -- underlining the diplomatic difficulties Rajoelina may face.
"South Africa and SADC will never countenance the unconstitutional transfer of power from a democratically-elected government in any of our member states," Motlanthe said.
Still, some analysts said the departure of Ravalomanana -- a 59-year-old self-made dairy tycoon -- would at least end the bloodshed for now and soothe the concerns of foreign investors.
"The transitional government will probably not take aim at foreign investors in the extractive industries, in part because it will be desperate for those revenues," said Philippe de Pontet, Middle East and Africa analyst at Eurasia group.
Madagascar has been enjoying a boom in mining and oil exploration, with a clutch of multinationals on the island.