ANTANANARIVO, April 30 (Reuters) - Malagasy police have arrested two opposition politicians for questioning after authorities broke up protests in a regional capital and arrested 20 people, a senior official said.

Protestors in Toliara, in the south-west of the enormous Indian Ocean island, demonstrated on Friday against power cuts, charcoal shortages and a ban on the sale of small fish but the event degenerated into vandalism and looting.

"Senator Robert Razaka and Deputy Dimby Benaria have been arrested," Colonel Rolland Randriamampionona, state representative in Toliara said by phone late on Sunday.

"They will be part of an enquiry into Friday's demonstration in Toliara," he said, adding that the legislators did not resist Sunday's arrests.

Jean Claude Tsiazy, a senior local government official, said the demonstration had not been authorised and "the forces of order were obliged to intervene".

"They fired teargas and shots in the air to disperse the demonstrators," he added.

Demonstration organisers said university students, not protestors, were responsible for the looting.

Observers of Madagascar welcome the energy and determination of President Marc Ravalomanana but question his authoritarian style.

On Friday, a senior opposition figure and former presidential candidate, Roland Ratsiraka, was detained in an eastern port town following accusations of corruption.

Ratsiraka, nephew of former President Didier Ratsiraka, came third in December's presidential elections with just over 10 percent of the vote.

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