Former rebel leader John Garang was sworn in as Sudan's first vice president as a new power-sharing constitution came into force in a bid to put a final seal on two decades of civil war.

Garang will also head an autonomous administration due to run the mainly animist and Christian south for a six-year interim period leading up to a promised referendum on independence.

Garang's inauguration came after President Omar al-Beshir formally promulgated the new constitution, bringing to an end the state of emergency in force on and off since he seized power in a 1989 coup and banned all political parties.

UN chief Kofi Annan hailed "a day of great hope for the Sudanese people, who have suffered so long."

The civil war, which was Africa's longest-running, left an estimated two million people dead and more than four million displaced.

But Annan warned that many challenges remained ahead if the new government was really to be one of national unity.

The UN chief expressed particular concern about the potentially destablising effects of continued ethnic minority uprisings in the western Darfur region, and Kassala and Red Sea states in the east.

Arab League chief Amr Mussa, US Deputy Secretary of State Robert Zoellick and several regional heads of state also attended the inauguration.

The authorities imposed tight security across the capital for the ceremony, declaring a public holiday and ordering motorists to stay away from the city centre.

PHOTO CAPTION

Former Sudanese rebel leader John Garang (L) shakes hands with Sudanese President Omar al-Beshir in Khartoum. (AFP)