Sudan's Justice Minister Mohamed Ali al-Mardi told Reuters on Sunday that charges had been filed against the 25 accused, which was why defence appeals for their release had been rejected.

Defence lawyers said the detainees must be formally charged or released.

Mardi said various charges had been filed against the men, including counter terrorism charges, illegal possession of arms and aummunition and conspiracy to commit criminal acts.

Among those held were opposition politicians Mubarak al-Fadil, head of the Umma Party for Reform and Renewal, and Ali Mahmoud Hassanein, deputy secretary-general of the opposition Democratic Unionist Party (DUP).

Moez Hadra, one of Hassanein's lawyers, said that according to Sudanese law any formal charges must be pressed in the presence of the defendant or their lawyer, so that any appeal can be made against the charges within a week to a higher judicial authority.

"That has not happened as yet and that is why we again today filed a petition to the authorities that they either charge the accused and transfer them to a court of law or release them," he said.

He said the defence was very concerned at the health of 71-year-old diabetic Hassanein, who was deteriorating on the sixth day of a limited hunger strike -- consuming only dates and water -- in protest at his continued detention.

"He is refusing to end the hunger strike despite many requests to do so and we are very worried about his health," Hadra said.

Fadil is also on hunger strike

The 25 were arrested at their homes four months ago, accused of attempting the overthrow the government and plotting to assassinate leading government figures.