The body of the Nigerian first lady, Stella Obasanjo, has arrived in the capital, Abuja, on a flight from Spain.
Stella Obasanjo was considered Olusegun's official wife
Mrs Obasanjo died in Spain on Sunday after undergoing plastic surgery. The cause of death is being investigated.
Her funeral will take place on Friday at President Olusegun Obasanjo's family compound in the town of Abeokuta.
Stella Obasanjo was recognised as Mr Obasanjo's official wife, although before coming to power in 1999, the president had several wives.
Correspondents say the death of the president's wife has caused huge speculation in Nigeria. She was not known to have any health problems.
The Molding Clinic at Puerto Banus, where Mrs Obasanjo underwent surgery, has issued a statement saying that her death did not take place during the operation, and that they were waiting for the results of a post mortem to establish the cause of death.
It did not specify the nature of the operation.
According to the Spanish news agency, EFE, Mrs Obasanjo had been recovering from the operation, which took place earlier in the week, when her condition suddenly deteriorated, and she went into a coma.
She was rushed to a hospital in nearby Marbella, but doctors were unable to save her.
EFE quoted judicial sources as saying that the post mortem had attempted to establish whether her death was linked to the operation - in which case the clinic could face a possible prosecution for malpractice.
Mrs Obasanjo, 59, was a controversial figure who ordered the arrest of newspaper publisher Orobosa Omo-Ojo earlier this year after his newspaper, the Midwest Herald, ran an article headed "Greedy Stella".
But as first lady, Mrs Obasanjo was also involved in child care projects and work with disabled people.
Books of condolence have been opened at the Banquet Hall of the State House in Abuja, the State House Marina in Lagos, and at the president's home in Otta.