Observers have argued that the regime needs to rally the opposition it can and operate a smooth transition to pull the rug out from under remaining rebel groups still engaged in fighting against Khartoum.

The conciliatory moves come as the International Criminal Court looks poised to indict members of Beshir's regime and its allies over a scorched-earth campaign to put down an uprising in the western region of Darfur.

Khartoum also faces a fresh rebellion in eastern Sudan, where armed tribes and opposition movements complaining of marginalisation are threatening to duplicate the chaos that has torn Darfur for more than two years.

Under domestic and international pressure, the field marshal is also seeking to start the new phase with a clean slate but the abrupt resurgence of his rivals makes for a complex and fragile political fabric, analysts said.

On the 16th anniversary of the military coup that took him to power, Beshir released Islamist leader Hassan al-Turabi, his eminence grise turned arch rival, once the chief ideologue of a now flagging regime.

Beshir also announced that all political prisoners would be freed and that a state of emergency which continued intermittently since his 1989 coup would be lifted.

Thursday's landmark announcements come quick on the heels of a series of unprecedented developments in Sudan, where parliament is reviewing a new constitution due to come into force on July 9.

This will coincide with the start of the six-year interim period of rule provided for in a landmark January peace deal that ended 21 years of deadly fighting between the mainly Arab Muslim North and the Christian South.

During that period, Africa's largest country will be jointly ruled by a unity government formed by according to quotas set in the peace agreement until elections are organised in three years.

At the end of the six-year period, the south will hold a referendum to decide whether to secede.

Khartoum also signed an agreement with the country's largest opposition bloc, the National Democratic Alliance (NDA), on June 18 that gave a boost to efforts to bring peace back to Africa's largest country.

Political analysts doubted Turabi would be eager to join the national unity government, but agreed that his release would further rattle the political landscape.

"I do not believe that he will want to be a partner in the government," said Sudanese columnist Mortada al-Ghali, arguing that deep differences between the former allies meant there was only a slim chance Beshir would reach out to Turabi.

The interim constitution being reviewed grants the lion's share of power to Beshir's ruling National Congress party and John Garang's former southern rebels, leaving opposition groups with a paltry 20 percent.

"But Turabi has enough room to manoeuvre," Ghali said. "He could be a threat to the National Congress."

The party has formed an alliance with one of Sudan's largest groups, the Umma party of former prime minister Sadiq al-Mahdi and nearly a dozen smaller political organizations.

Turabi's Popular Congress party still enjoys massive support among Islamist movements, students and paramilitTary forces that he and Beshir created after the 1989 coup, according to Ghali.

Turabi celebrated his first day as a free man by criticising the new interim constitution and attacking the government's record on freedoms.

"For the first time in the world, a constitution determines the majority that will rule the country while this should be left to the people to determine," he said.

"Freedoms are still forbidden," Turabi charged, complaining about press censorship, restrictions on political parties and bans on street demonstrations.

Ghali said while Beshir was indeed striving to unite Sudanese ranks ahead of the interim period, the January peace agreement required anyhow that he free political prisoners and lift the state of emergency.

"He wanted to make it look like a political gesture on his part," he said.