KHARTOUM - Rights group Amnesty International said Sudan had tortured five people detained since July on accusations of planning to overthrow the government, and one needed urgent medical help.

"The five men are held at Kober Prison in the capital, Khartoum, where they have been tortured," Amnesty said in a report seen by Reuters on Saturday.

Sudanese officials arrested a number of former army officers and opposition leaders in July, accusing them of a plot to overthrow the government and when that failed, of seeking to create chaos in the capital.

A Sudanese national security spokesman denied the men had been tortured.

Mubarak al-Fadil, head of the opposition Umma Party for Reform and Renewal, his party secretary-general Abdel Jalil al-Basha and the deputy head of the large opposition Democratic Unionist Party (DUP) are among those being held. None of the detainees has been charged.

Amnesty said one DUP member, Ahmad Salman, had been severely tortured to extract a confession and was in need of urgent help.

"Ahmad Salman needs urgent medical attention after severe and prolonged torture carried out in an attempt to persuade him to implicate Mubarak al-Fadil... in an alleged armed coup attempt," the report said.

Amnesty said many of the detainees reported being subjected to brutal torture techniques while under interrogation.

"Torture methods reportedly include being suspended from the ceiling by their wrists, and tying up the victim and beating him with hosepipes . . . Another said that his testicles were crushed; they remain swollen and he needs medical attention," the report added.

Amnesty said the five had been threatened with further torture.

The family of opposition leader Fadil said he was granted urgently needed medical attention last month after more than 40 days in detention.

Last week the legal committee overseeing Fadil's case rejected an investigative report submitted by his defence team, which requested Fadil's release on the grounds that there was no evidence to link him to the coup plot.

"The legal committee said it was not the appropriate timing. So they (Fadil's lawyers) are going to submit the legal document again this week as requested," Fadil's daughter, Habab Mubarak told Reuters.

The opposition said last month it was concerned authorities were targeting opposition parties ahead of elections due by the end of 2009.

U.N. special rapporteur on human rights in Sudan, Sima Samar, has expressed concern at the arrests ahead of the elections and urged the government to work with more transparency on the matter.