The Guinea Bissau government has moved to calm fears sparked by the announcement that former President Kumba Yala says he is still in charge.
Yala said he had been forced to sign a letter of resignation
Mr Yala declared himself leader two years after being ousted in a coup.
Elections are due in June to replace the transitional administration which has ruled since Mr Yala's removal and he was due to stand.
His declaration caused some observers to question whether the elections would take place.
Guinea Bissau has been plagued by coups and chaos since independence from Portugal in 1974.
Prime Minister Carlos Gomes Junior, currently visiting Lisbon, told a Portuguese radio station he had confidence in guarantees received from army leaders.
"The military has assured me that all was calm, that they would not use force," the prime minister said.
Defence Minister Martinho Ndafa Cabi said there was no reason for alarm as Mr Yala was just "a common citizen", but security was stepped up.
Mr Yala is believed to have some support in the army, which is dominated by members of his Balanta ethnic group, and he was chosen last month by the largest opposition group, the Social Renovation party, as its presidential candidate.
Speaking to reporters outside his home in the capital, Bissau, Mr Yala announced that he was withdrawing a letter of resignation which he said he had been forced to sign after the military coup in 2003.
"I have decided to revoke my renunciation of power and to retake power as early as today, 15 May, as the elected president," he announced
He said he would serve out the remaining 18 months of his five-year term of office, which began in 2000 - and elections may be delayed.
He said he had decided on this course of action as there had been "a political vacuum" in the country for the last week, and he went on to accuse the current interim president and prime minister of plotting to hold on to power.
In New York, UN Secretary-General Kofi Annan said he was "deeply concerned" at statements apparently designed "to disrupt the ongoing transitional process".
Another former president, Nino Vieira, returned home to a hero's welcome last month after six years of exile in Portugal.
He took power after a coup in 1980, and was ousted in 1999.
Both former leaders are seen as the front runners in any presidential election.
The Supreme Court ruled last week that both men could contest the polls.