Opposition groups in Nigeria say they are going to challenge the appointment of the new military leader, General Abdulsalam Abubakar, in court.

Watch Gen Abubakar at his swearing-in ceremony

They have described his appointment as illegal, arguing that the army's provisional ruling council - the body which appointed him - has no legal basis for such a decision.

The umbrella opposition group, the Joint Action Committee, says it will take the issue to the Nigerian High Court next week.

Earlier, there had been widespread calls for the release from jail of the man presumed to have won the 1993 presidential election, Chief Mashood Abiola.

In his first address to the nation after the death of his predecessor, General Abubakar promised to press ahead with plans to return Nigeria to constitutional rule, but he gave no details.

He appealed to all Nigerians in exile abroad to return home to join the process of national reconstruction and reconciliation.

In a television broadcast on Tuesday he said: "We shall need the full co-operation of all Nigerians to succeed in this sacred endeavour.

"We wish to extend invitations to all those Nigerians in self exile to return home to join the process of reconstruction, reconciliation and conclusion of the transition programme."

One leading human rights campaigner, Gani Fawehinmi, said the new leader's commitment to the programme of his predecessor, General Sani Abacha, showed he had nothing to offer.

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

A BBC correspondent in Lagos says many people will view General Abubakar's decision to extend the period of national mourning to a month as decidedly odd, given that General Abacha's death was a source of celebration for some Nigerians.

The correspondent said the new leader gave one thing away in his inaugural speech when he appealed to Nigerians that he described as in 'self-exile' to return home to join a process of national reconciliation.

Abubakar addresses Nigeria: "sacred endeavour"

Some Nigerian pro-democracy activists, who were forced to flee the country for fear of arrest by the military may well judge this description of themselves as an insult.

The correspondent says the test of transition will be whether there is a free choice of candidates or if pressure is applied by the military.

Another senior member of the ruling military, Air Vice-Marshall Isaac Alfa said the government would stick with the October 1 date fixed by General Sani Abacha for the restoration of a civilian government.

But he added that the date for presidential elections, currently August 1, may change.

Wole Soyinka: 'This transition government was dead the day it was born'

After General Abacha promised a democratic transition, the five political parties involved in the process were hand-picked and their leadership vetted by the government.

All five went on to choose General Abacha as their sole presidential candidate.

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