* Security tightened in capital Libreville

* Three rivals claim victory, authorites say 'speculation'

* Election office to announce winner on Wednesday

(Adds interior minister quotes)

By Linel Kwatsi

LIBREVILLE, Sept 1 (Reuters) - Gabonese security forces took up positions at key points in the capital on Tuesday as election officials further delayed announcing the winner of Sunday's hotly contested presidential vote.

Three rival contenders, including the son of President Omar Bongo who died in June, have claimed victory in the election in the central African oil producer, but the authorities have dismissed their estimates as speculation.

"The figures doing the rounds do not in our view have any firm basis," Rene Aboghe Ella, president of the election commission, told reporters, adding that the commission would meet to verify the results on Wednesday.

Opposition leaders accuse ex-defence minister Ali Ben Bongo of rigging the poll to ensure a dynastic succession to his late father, but election monitors and ex-colonial power France have said they are broadly satisfied with procedures so far.

"The mission calls on the candidates ... and the entire population to ensure peace and democracy is maintained in Gabon by sticking to dialogue," Albert Tevoedjre, head of the AU election observer team, said on Tuesday.

Omar Bongo's death in June, aged 73, ended nearly 42 years of rule that imposed stability on Gabon but gave rise to allegations that he lavished petrodollars on family and friends but failed to alleviate widespread poverty.

Ben Bongo, 50, had long been the favourite and was quick to declare victory on Monday. His claim was disputed by former interior minister Andre Mba Obame and by Pierre Mamboundou, one of the few rival candidates with no ties to Bongo. Two Ben Bongo rivals also claimed victory.

A Reuters witness said riot police had been sent to Libreville's Place de la Paix, one of the squares often used for political gatherings, and Republican Guard soldiers had joined the usual gendarmes outside some government buildings.

"We are not afraid of anything ... we are taking steps to ensure people's security so they can get on with their lives," Jean-Francois Ndongou, interior minister and acting defence minister, told reporters on Tuesday.

DEMOCRACY TEST

Ndongou said the measures were not meant to restrict movement and gatherings were allowed at home or in certain places, but demonstrations must be approved by the government.

"It is wait-and-see for the moment," said strategist Ridle Markus at Absa Capital Research of the investment outlook.

"There is no real risk or reason why they (investors) should get out. But at the same time we would be advising them not to go in until there is clarity about the result and in particular the reaction to the result," he added.

Gabon's electoral system allows for only one round of voting, meaning that a candidate need only win more votes than his nearest rival rather than the majority of the ballots cast.

Some analysts have said this clearly favoured Ben Bongo as his more than 20 rivals struggled to unite until a handful dropped out to back Obame two days before the vote.

Gabon hosts oil firms including France's Total and U.S.-based Vaxalco, and is one of the few sub-Saharan countries to have launched a Eurobond.

The vote is the latest democracy test in a region that has seen a string of disputed polls and power grabs over the past year, including the move by Niger's president to extend his term in office, and a disputed election in Congo Republic.

Some fear a transfer of power from Bongo father to son would encourage other elderly African leaders to groom their offspring for future rule, for example in Senegal and Libya where leaders' sons already hold senior positions.

Analysts say Bongo's successor will have to cope with dwindling oil reserves and falling revenue from the sector, which currently makes up half of national output. (Writing by Mark John and David Lewis; editing Tim Pearce)

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