Cipriano Cassamá, one of the two men declared president of Guinea-Bissau, has resigned from the post after just one full day in office, saying his life was in danger.
This was despite the fact that former army general Umaro Cissoko Embaló had already been sworn in as president at a hotel in the capital, Bissau.
Guinea-Bissau has had nine coups or attempted coups since 1980.
The national electoral commission declared that Mr Embaló had beaten his main rival, Domingos Simoes Pereira, by 54% to 46% in the 29 December run-off election.
Outgoing President José Mário Vaz handed power to Mr Embaló at a ceremony at a luxury hotel on Thursday.
The poll was intended to draw a line under the past, but it has triggered a new political crisis in a nation where the military wields huge political influence.
But Mr Pereira's PAIGC party, which led Guinea-Bissau to independence and was the only legal party until 1990, rejected Mr Embaló's inauguration, saying the election was marred by fraud.
It then used its parliamentary majority to swear in Mr Cassamá, the parliamentary speaker, as interim president, until the Supreme Court ruled on its bid to annul Mr Embaló's victory.
Mr Cassamá said he had no choice but to give up the post because he had received death threats.
"I have no security... My life is in danger, the life of my family is in danger, the life of this people [nation] is in danger. I cannot accept that, that is why I took this decision," he told reporters.
As to who threatened Mr Cassamá's life, he did not reveal.
Guinea-Bissau still has two rival prime ministers.
On Sunday, one of them, Aristides Gomes, said military officers had invaded his private home in Bissau in a bid to force him to resign.
"They threatened to kill the security officers in my service if they do not resign, and threatened to kill me if I do not resign from the office to which I was legally appointed," he said.
Mr Gomes has refused to recognise a decree by Mr Embaló, which replaced him with Nuno Gomes Nabiam, a former presidential candidate who threw his weight behind Mr Embaló in the run-off election.
The West African regional bloc, the Economic Community of West African States (ECOWAS), has called for an end to the turmoil, and has warned that a military force is on standby to "re-establish order" in the event of a coup.
Mr Embaló has said he wants to resolve tensions and modernise Guinea-Bissau - one of the world's poorest nations, which is home to some 1.6 million people
