* Hery Rajaonarimampianina won 53.5 percent of vote
* Rigging skewed poll, says defeated candidate
* Dispute risks prolonging political paralysis (Adds defeated candidate and French ambassador)
By Alain Iloniaina
ANTANANARIVO, Jan 17 (Reuters) - Madagascar's electoral court declared former finance minister Hery Rajaonarimampianina president-elect on Friday, an outcome his defeated rival swiftly rejected.
The ruling raises the spectre of a prolonged row over the result of the Dec. 20 vote, the first since a coup on the Indian Ocean island in 2009, extending a political crisis that has sharply slowed economic growth and deepened poverty.
"I urge goodwill from everyone so that we can build a prosperous and stable nation," said Rajaonarimampianina, who was backed by the outgoing president, Andry Rajoelina, who spearheaded the 2009 overthrow of Marc Ravalomanana.
His opponent Jean Louis Robinson, who alleges widespread rigging of the vote, said he would not accept the result and would continue to challenge the outcome.
"We contest the court's decision in the strongest way," he told reporters, after boycotting the formal announcement.
The electoral court said Rajaonarimampianina won 53.5 percent of the vote to Robinson's 46.5 percent, confirming the electoral commission's provisional results.
Robinson, who was backed by Ravalomanana, said he would not yet be calling on his supporters to protest on the streets of a country that has seen years of political turmoil, sometimes violent.
He plans to outline the vote's "irregularities" to the Southern African Development Community and African Union. Both blocs had worked on a political deal to push Madagascar towards an election.
A drawn-out dispute is likely to stir up further the nickel-producing island's volatile political scene and could delay restoring the external budget support needed to spur public spending and growth.
The vote was meant to end a crisis that has driven out investors, cut aid flows and led to the diplomatic isolation of the former French colony.
French Ambassador Francois Goldblatt said the court's verdict marked a step in the restoration of constitutional order on the world's fourth-biggest island.
His comments suggested donors might be edging closer to resuming direct aid that accounted for 40 percent of Madagascar's budget before the political crisis began.
"Everyone should accept the court's result," said mechanic Faly Ranarison before Friday's ruling.
"Let the country be peaceful and wait for the next election to set the record straight."
The streets of the capital Antananarivo, where Rajaonarimampianina struggled to win support in the first round of voting, were calm after the court's announcement, but some worried about the risk of unrest.
"I'm worried," said teacher Noro Ravaonirina. "Recently there's been talk, right or wrong, about vote-rigging. In such an environment you can't exclude that the loser won't accept his defeat easily." (Writing by Richard Lough; Editing by Edmund Blair and Andrew Roche)