(Releads with state security official)

By Opheera McDoom

KHARTOUM, July 15 (Reuters) - Sudanese authorities have arrested 17 people including opposition leader Mubarak al-Fadil and retired military officers, accusing them of trying to overthrow the government, state security leaders said on Sunday.

Fadil and Abdel Jalil al-Basha, both of the Umma breakaway party, were arrested in the early hours of Saturday morning. Fadil's party denied the accusations.

"Their plan was to overthrow the government, but it was very weak," Mohamed Abdallah Atta, deputy head of Sudan's state security organ, told reporters. "They were ready to fight for seven days," he added.

He said the group had wanted to execute their plan on July 15 but, lacking arms and soldiers, they had decided to delay and meet again on July 20. Lacking support, they planned instead to cause chaos in the capital.

Atta said the group planned to bring 1,000 conscripts from South Kordofan state in central Sudan, but only 30 had arrived in the capital.

He said Fadil was the group coordinator and was now in Kobar prison, but others were still being questioned by state security. Those charged would be taken to court, he added.

Atta said contacts had been made with undisclosed foreign countries, but said no neighbouring Arab country was involved.

The conspirators made calls to rebel groups from Sudan's war-torn Darfur region, requesting more weapons, he said.

Fadil broke from the popular Umma party led by his cousin, ex-Prime Minister Sadig al-Mahdi, in 2002 and joined the National Congress Party government as a presidential adviser. Basha was tourism minister.

Fadil was sacked in 2004, his breakaway Umma party splintered and he returned to opposition politics.

"It is obvious for us that this is an attempt to frame him and distract people from problems on other fronts," a statement from Fadil's party said. "The party always renounced violence and always called for democratisation," it added.

Ali Mahmoud Hassanein, a politician from the opposition Democratic Unionist Party (DUP), was also arrested but later released without charge. The DUP and Umma are considered the two largest northern political parties.

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

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