Nigeria’s Vice-President Goodluck Jonathan has become acting president, after weeks of political turmoil caused by the absence of its ailing leader.
In a televised address to the nation, Mr Jonathan described the role as a “sacred trust”.
The National Assembly earlier voted to recognise him as acting leader in place of President Umaru Yar’Adua.
Mr Yar’Adua has not been seen in public since going to Saudi Arabia in November for medical treatment.
His continued absence has sparked legal challenges, cabinet splits and mass street protests.
It has led to a freeze in government business and threatened progress made in combating unrest in the oil-rich Niger Delta.
“I am fully aware of the responsibilities reposed in me, and I want to reassure all Nigerians that this is a sacred trust, which I shall discharge to my fullest abilities,” Mr Jonathan said in his televis ed address on Tuesday.
In a country with a history of military coups, he thanked the armed forces for “their loyalty and devotion to duty during this trying period”.
Mr Jonathan, due to chair his first cabinet meeting later, said he would build on the amnesty for Niger Delta militants begun by Mr Yar’Adua.
Some militants in the region have threatened to break the amnesty, citing government delays in implementing the programme.
He asked the country to pray for Mr Yar’Adua’s recovery.
“The circumstances – in which I find myself assuming office today as acting president of our country – are uncommon, sober and reflective,” he said.
“More than ever therefore, I urge all Nigerians as a people who have faith in God to pray fervently for the full recovery of our dear president, and his early return.”
Mr Yar’Adua is said to be receiving treatment for an inflammation of the lining around the heart and he has long suffered from kidney problems.
Earlier on Tuesday, both houses of the National Assembly passed a motion for Mr Jonathan to take over as head of state until Mr Yar’Adua is able to resume the presidency.
The BBC’s Ahmed Idris in Abuja says the move among the political elite to back the vice-president is unprecedented.
The ruling party alternates leadership between north and south, and Mr Yar’Adua’s northern backers had wanted to keep his southern deputy out of office.
Executive power is transferred to the vice-president under Nigeria’s constitution when the president formally informs parliament of his absence.
Mr Yar’Adua never did this, but senate leader David Mark said that an interview Mr Yar’Adua carried out with the BBC’s Hausa service on 12 January had effectively provided the assembly with the notice it needed.
But some analysts have suggested that the assembly’s motion is not legally binding and could face court challenges.