Sadiq al-Mahdi said he wanted to solve Darfur's problems
Former Sudanese Prime Minister Sadiq al-Mahdi has announced he will stand against current leader Omar al-Bashir in a forthcoming presidential election.
Mr Mahdi was elected in Sudan's last multi-party vote in 1986, but was overthrown by Mr Bashir in a 1989 coup.
The BBC's James Copnall, in Khartoum, says Mr Mahdi's presence gives real legitimacy to April's election.
The polls are part of a 2005 peace deal that ended a two-decade civil war between north and south Sudan.
Some of the other major figures in Sudanese politics, such as southern leader and national Vice-President Salva Kiir and Islamist leader Hassan al-Turabi, have opted not to challenge President Bashir.
The parties of both men are instead fielding less well known candidates in the presidential election.
Mr Kiir is standing for re-election as the south's leader, ahead of a referendum on whether the region should seek independence from the rest of Sudan, due in 2011.
In announcing his candidacy, Mr Sadiq referred to his ousting by Mr Bashir in a bloodless coup.
"I have not been fired by the people, I have been fired by the guns," he said.
"Now it is possible for the people to reinstate whom they believe represents their interest, represents aspirations."
He also raised the possibility of ending the troubles in the Darfur region - where thousands of black Africans have been killed by Arab militias.
"Darfur can be resolved if there is sufficient political will, if there is sufficient political leadership," the 74-year-old said.
Mr Bashir, who is widely expected to win the election, is wanted on an international arrest warrant for war crimes in Darfur.
His government denies accusations that it is backing the militias who have carried out mass murder.