Regional leaders are visiting the West African state of Guinea-Bissau in an effort to defuse the crisis following the assassination of the president.
Guinea-Bissau's parliament has elected the speaker as the West African state's interim leader, a day after the president was assassinated.
Soldiers killed Joao Bernardo Vieira on Monday in an apparent tit-for-tat attack after army chief-of-staff Gen Tagme Na Waie was blown up.
Raimundo Pereira must organise a fresh presidential election within 60 days.
West African regional group Ecowas is due to hold an emergency summit about the crisis in the capital, Bissau.
Diplomats representing West African and Portuguese-speaking nations have been meeting the government to ensure the constitution is honoured.
The city reportedly remains calm in the aftermath of the double assassination.
President Joao Bernardo Vieira was shot dead by soldiers who blamed him for a bomb attack that killed the army chief.
Bizarre twist
Guinea-Bissau - a major transit point for Latin American cocaine headed for Europe - has been plagued by political unrest since it gained independence from Portugal in 1974.
The British consul in Bissau, Jan van Maanen, told the BBC's Network Africa programme the capital was deserted.
"There's no traffic, there's nobody on the street at all actually," he said. "There's no military on the streets, there's no check-points."
In a bizarre twist, it has emerged British novelist Frederick Forsyth was in Guinea-Bissau as the president was assassinated. He described the drama, which could have leapt straight from the pages of one of his thrillers.
Before taking the oath of office on Tuesday, national assembly leader Mr Pereira said Guinea-Bissau was "facing a very delicate situation".
The Day of the Jackal author told the BBC's World Today programme he had heard that soldiers first threw a bomb through the president's villa and the blast caused the roof to collapse.
He called on MPs "to assume their responsibility toward the nation".
Mr Vieira had emerged alive from the rubble and was shot by his assailants but still did not die, said the writer, who was in the country to research a new book.
Lawmakers in the capital Bissau also observed a moment of silence for President Vieira and for the army chief-of-staff Gen Tagme Na Waie.
The soldiers then took the wounded president to his mother-in-law's house where they "chopped him to bits" with machetes, according to Forsyth.
Life in the city began to return to normal with some businesses re-opening, although shoppers briefly fled two market places in panic after rumours spread of fresh shooting, according to Reuters news agency.
'Island exile'
'Fragile situation'
Hours earlier a bomb had killed the army chief at his headquarters. The device was reportedly hidden underneath the staircase leading to Gen Waie's office.
Nigerian President Umaru Yar'Adua, who heads the 15-member regional bloc Ecowas, has sent a delegation to the country.
Guinea-Bissau - a major transit point for Latin American cocaine headed for Europe - has been plagued by coups and political unrest since independence from Portugal in 1974.
Gen Tagme Na Waie was once reportedly exiled to a desert island
Soldiers killed President Vieira early on Monday in an apparent tit-for-tat attack after Gen Waie was blown up in his headquarters hours earlier.
The African Union's Peace and Security Council met on Tuesday in the Ethiopian capital Addis Ababa to discuss their response to the crisis.
The African Union's Peace and Security Council decided not to suspend Guinea-Bissau when it met in the Ethiopian capital Addis Ababa on Tuesday, as it said the attacks did not represent a coup.
According to the AU's statutes, member states should be suspended in the event of an unconstitutional power-change, as were Mauritania and Guinea after coups last year.
The AU suspended neighbouring Guinea and Mauritania after coups last year.
Ecowas Chairman Mohamed Ibn Chambas told the BBC five West African foreign ministers were joining him in Guinea-Bissau and they would seek to ensure the army does not seize power.
Guinea-Bissau's army has denied it is launching a coup and has promised to honour the constitution.
He said: "We see these two [deaths] as certainly a step backwards, and we will remain resolved in the region and ensure that constitutional rule in Guinea-Bissau is respected."
Portugal's foreign minister also arrived in Bissau and reportedly said he did not see any need for international military intervention in the country.
The European Union and former colonial ruler Portugal have also called for the rule of law to be respected.
Senegalese President Abdoulaye Wade has reportedly sent a plane to pick up Mr Vieira's wife and children and take them to Dakar.
The army has denied it is launching a coup and promised to honour the constitution.
Guinea-Bissau, an impoverished former Portuguese colony of 1.6 million people, has been plagued by coups since 1980, when Mr Vieira himself first came to power in one.
Monday's attack was the second on President Vieira in recent months.
Under the charter, the speaker of parliament, Raimundo Pereira, has now taken office and must arrange elections within 60 days.
In November, his residence was targeted by soldiers with automatic weapons.
The cabinet has announced seven days of national mourning for both leaders and launched a judicial inquiry into the deaths.
Braima Camara, a reporter from privately-owned Radio Pindiquiti in Bissau, told the BBC that Senegalese President Abdoulaye Wade had sent a plane to pick up Mr Vieira's wife and children and take them to Dakar.
Monday's attack was the second on President Vieira in recent months. In November, his residence was attacked by soldiers with automatic weapons.
New details have emerged of the enmity between the two men, who had been his political rival for decades.
In January, Gen Waie had a narrow escape when unidentified gunmen opened fire on his car.
In January, Gen Waie had a narrow escape when unidentified gunmen opened fire on his car.
He reportedly suspected the attack had been ordered by Mr Vieira as he had just stepped outside after receiving a call from the presidency asking him to come at once.
He reportedly suspected the attack had been ordered by Mr Vieira, as he had just stepped outside after receiving a call from the presidency asking him to come at once.
Gen Waie was reportedly among majority ethnic Balanta officers suspected of plotting a coup and punished in the late 1980s by the president, who came from the minority Papel ethnic group.
Mr Vieira exiled the army chief for a number of years to a deserted island off the coast of Guinea-Bissau before he was officially pardoned and allowed to return, Gen Waie's chief-of-staff, Lt Col Bwam Namtcho, told AP news agency.
