Nigeria's new chief justice was sworn-in Wednesday by his predecessor instead of by President Umaru Yar'Adua, who is in hospital in Saudi Arabia, sparking claims his appointment is illegal.

Aloysius Katsina-Alu, 68, took the oath of office before the outgoing Idris Kutigi, who retires on Thursday.

Traditionally, the head of state swears-in the chief justice but Yar'Adua has been receiving treatment for a heart condition in a Jeddah hospital for more than a month.

Lawyers say having Kutigi swear-in his successor is illegal.

"The whole exercise of swearing in a new justice by the outgoing one is a huge joke and an illegality," prominent Nigerian lawyer Femi Falana told AFP.

"The outgoing chief justice has no right under the law to swear-in his predecessor he did not appoint," he added.

Falana said that because the legality of the ceremony was being questioned, it was open to a court challenge.

Kutigi defended the move, insisting it was provided for under law.

"It is happening for the first time. It is not because it is the fault of anybody but because the law has always been there," Kutigi said.

"The swearing-in of Chief Justice of Nigeria is either by Mr President or the outgoing or retiring chief justice but we haven't had an occasion in the past. Now the occasion has arisen," Kutigi said.

Commentators have said Yar'Adua's long absence from the country is stalling government business as his deputy has not been constitutionally empowered to act, prompting opposition calls for his resignation.

Justice Minister Michael Aondoakaa told a federal high court in Abuja Tuesday that Vice President Goodluck Jonathan could not become the nation's acting president because Yar'Adua has neither resigned nor allowed his deputy to take over.

He said Yar'Adua was not suffering from an infirmity that would render him permanently incapable of discharging his functions.

On Wednesday, his spokesman Olusegun Adeniyi said the president had signed a piece of urgent legislation, the 2009 supplementary budget, at his hospital bed.