A student facing a defamation case for a tweet about Nigeria's first lady had the charges against him dropped Friday, his lawyer said.

Aminu Adamu, born in 1998, had been charged with "criminal defamation" and arraigned over a tweet about the physical appearance of Aisha Buhari, the wife of President Muhammadu Buhari.

But on Friday, his lawyer and the prosecutor said the charges had now been dropped.

"The first lady has withdrawn the charges," defence counsel Chiejoke Kingsley Agu told AFP, without giving details.

A police officer prosecuting the case told AFP that Adamu's charges had been dropped at the insistence of the president's wife.

"In fact, the defendant will be released from custody any moment from now. The documents for his release are being processed," said the officer, who asked not to be identified.

Adamu's arrest and prosecution sparked condemnation from Amnesty International, local rights bodies and activists, who called for his immediate release.

Felix Atah, The president of National Association of Nigerian Students (NANS) who had called for a nationwide protests over Adamu's arrest, welcomed the news that the case had been dropped.

"We shall personally take him back to school as soon as he is freed so that he can write his final examinations which begin on Monday," he said.

- Twitter suspension -

Adamu was arrested at his university in northern Jigawa state on November 18 and transferred to the capital Abuja, five months after his online post about Aisha.

He was detained after the first lady filed a complaint, triggering a police investigation, according to court documents seen by AFP and a judicial source who asked to remain anonymous.

Based on the results of that preliminary investigation, the capital's police commissioner filed a lawsuit against the student.

According to the court documents, Adamu said he had commented on a photo of the first lady on Twitter. It was not immediately clear whether he had intended to make a joke or accuse her of corruption.

It was not the first time that a post on Twitter has caused uproar among the country's political class -- the platform was suspended for seven months last year.

Allegations of mistreatment against detainees are not uncommon in Africa's most populous country, despite it having had a stable democracy since 1999 after decades of military dictatorships.