Bazoum has been detained by the ruling military since he was ousted in a coup last July.

The Niger authorities accuse Bazoum of treason, financing terrorism and plotting to undermine the state.

In a statement sent to AFP on Friday, Bazoum's lawyers urged ECOWAS's heads of state to "demand the immediate release of the president and his wife Hadiza on the eve of the summit" -- to be held on Sunday in Abuja.

"If it had not been for the unconstitutional overthrow in Niger nearly a year ago, president Bazoum would be sitting among the heads of state meeting in Abuja this weekend," said lawyer Mohamed Seydou Diagne in the statement.

Bazoum has been held at the presidential residence with his wife since the coup on July 26.

The statement pointed out that Bazoum and his wife "have never been brought before a magistrate and have not been informed of the existence of any proceedings against them."

The statement added that: "Since 19 October, they have been completely cut off from the outside world and no one has been authorised to visit them, apart from their doctor."

The deposed president's immunity was also lifted in mid-June by Niger's highest court.

"The situation is very serious," said Moussa Coulibaly, another lawyer for the ousted president.

"It is the responsibility of the ECOWAS authorities to ensure that their peer, the democratically elected president, is not forgotten, does not languish in arbitrary detention and, above all, does not pay with his life for his commitment to the rule of law."

Niger has been ruled by military leaders since the overthrow of elected president Bazoum, which prompted international condemnation.

Military ruler General Abdourahamane Tiani has said his regime wants up to three years for a transition back to a civilian government.