Watch Gen Abubakar at his swearing-in ceremony

A new leader of Nigeria has been sworn into office in a rapid transition after the unexpected death of General Sani Abacha.

The new head of state in Africa's most populous nation is the defence chief, Major-General Abdusalam Abubakar.

He was chosen by Nigeria's Provisional Ruling Council at a late-night session after the burial of General Abacha, who died suddenly in the early hours of Monday, leaving a potential power vacuum.

Nigeria's main opposition group, the United Action for Democracy, has rejected the appointment and has said it will go ahead with demonstrations on Friday calling for the end of military rule.

A leading dissident, the Nobel laureate Wole Soyinka, said the world must put pressure on Nigeria to enact democratic reforms.

Gen Abacha's body being taken out of his residence in the capital, Abuja

In a brief acceptance speech to the ruling military council in Abuja, General Abubakar, a career soldier, described his appointment as a personal challenge.

"All hands must be on deck to move this nation forward. I will address the nation in due course," he said.

The new head of state made no mention of presidential elections looming on August 1 for which his predecessor was the sole candidate.

Nigerian army rulers have repeatedly promised and repeatedly delayed the restoration of democratic rule.

Correspondents say General Abubakar will have to decide whether to hold presidential elections planned for August, and whether to release political opponents of Mr Abacha.

The new leader declared a week of national mourning from Tuesday in honour of Abacha. Flags will be flown at half mast on all public buildings.

General Abacha, who reportedly died of a heart attack, left no constitutional replacement.

His former deputy, Lieutenant-General Oladipo Diya was sentenced to death in April for plotting to take power, and the post was never filled.

Opposition calls

BBC West Africa correspondent Mark Doyle on neighbouring countries's fears of instability in Nigeria

Some opposition supporters have already said power must go to Chief Moshood Abiola, the presumed winner of the 1993 election, who has been in jail for five years awaiting trial for treason.

But the chairman of the opposition National Democratic Coalition of Nigeria (NADECO) said that whoever succeeded General Abacha should respect the scheduled transition to civilian rule.

Promises of civilian rule

BBC Lagos reporter Sola Odunfa: "opposition should be heartened by appointment."

Sani Abacha, who seized power in November 1993, had pledged to hand over to a civilian government. Two months ago, he became the only nominated candidate for the presidential elections on August 1 and was expected to hold on to power.

Government members portrayed him as the only man who could unite such a diverse nation.

But his opponents in Nigeria saw him as part of a military elite desperate to hang on to power.