Opposition candidate Fernando Dias da Costa told AFP he believed he had won Sunday's election and alleged outgoing President Umaro Sissoco Embalo -- who has also claimed victory -- had "organised" the power grab to prevent him taking office.

The coup struck just one day before authorities had been due to announce the provisional results of the presidential ballot and parliamentary polls.

General Horta N'Tam, chief of staff of the army, was designated the country's new leader for a period of one year.

He took the oath of office at the military's headquarters on Thursday, declaring: "I have just been sworn in to lead the High Command."

N'Tam is considered to have been close in recent years to Embalo, whom he now replaces.

Dias, who said he was safe and in hiding, was Embalo's main challenger after the main opposition candidate, Domingos Simoes Pereira, was barred by the supreme court from standing.

"I am the president (elect) of Guinea-Bissau," Dias told AFP by telephone, adding that he thought he might have garnered around 52 percent of the vote.

"There wasn't a coup," he alleged. It was "organised by Mr Embalo".

Analysts consulted by AFP also alleged the coup could ultimately benefit Embalo.

Dias said he had escaped from his campaign HQ on Wednesday when armed men came to arrest him.

Pereira, who backed Dias after being excluded from the electoral race, was himself arrested on Wednesday.

- 'Necessary measures' -

Bissau, capital of the west African country, was at a standstill on Thursday, AFP journalists observed.

Most shops and markets were closed and soldiers patrolled the streets.

The new military leaders banned "all media programming" and outlawed protests.

Surrounded by heavily armed soldiers, N'Tam told a press conference on Thursday the military had acted "to block operations that aimed to threaten our democracy".

N'Tam said evidence had been "sufficient to justify the operation" adding that "necessary measures are urgent and important and require everyone's participation".

General Denis N'Canha, head of the presidential military office, told journalists the army was assuming control "until further notice" after a plan involving "drug lords" had been uncovered, including "the introduction of weapons into the country to alter the constitutional order".

Land, air and sea borders -- which were all sealed off on Wednesday -- were reportedly reopened, however.

A nationwide curfew was lifted and the High Command ordered the "immediate reopening" of markets, schools and private institutions.

- 'Grave violation' -

Members of Guinea-Bissau's diaspora and researchers told AFP they queried the true motives behind the power grab, which they alleged could ultimately benefit Embalo.

Researchers interviewed by AFP said unverified preliminary results circulating before the coup showed opposition candidate Dias as the winner of the election.

"I doubt the veracity of the allegations put forward by the High Military Command regarding the restoration of order," one West African researcher told AFP on Thursday on condition of anonymity.

"This is a coup aimed at preventing the opposition candidate, Fernando Dias, from seizing power," he said.

"This is the ideal scenario for Mr Embalo, who could, following negotiations, be released and potentially reposition himself for the next elections."

The African Union on Thursday condemned the coup and demanded Embalo's immediate and unconditional release, while the chair of West African regional bloc ECOWAS called the affair a "grave violation of Guinea-Bissau's constitutional order".

The European Union urged "a swift return to the constitutional order and the resumption of the electoral process", adding: "The results of the presidential and legislative elections should be respected by all parties."

ECOWAS chair Julius Maada Bio, the president of Sierra Leone, said he was consulting fellow heads of state and government to discuss the "current political situation".

Sandwiched between Guinea and Senegal, Guinea-Bissau has experienced four putsches since independence from Portugal in 1974, as well as multiple attempted coups. Its election results are often contested.

West Africa has seen several coups in recent years, with Mali, Burkina Faso, Niger and Guinea all seeing their governments toppled.

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