The Umma Party, widely considered to be Sudan's largest political party and led by the last democratically elected prime minister of Sudan, Sadiq al-Mahdi, will lead the opposition coalition with Islamist Hassan al-Turabi's Popular Congress Party (PCP), despite previous enmity between the two.
"We are all for democratic change and for a truly national and comprehensive peace deal," Umma Party spokeswoman and daughter of Sadiq al-Mahdi, Mariam al-Mahdi, said on Thursday.
She said this was a political declaration rather than an agreement between the parties. The parties signed the declaration late on Wednesday.
A new constitution
The ruling National Congress Party and the former southern rebel Sudan People's Liberation Army (SPLA) in January signed a peace deal to end more than two decades of civil war in the south. The two parties are now drafting a new constitution and hope to form a new coalition government by 9 July.
The first round of talks between representatives from the national alliance and the SPLA's leader John Garang concluded in Cairo, Aljazeera reported.
"We are all for democratic change and for a truly national and comprehensive peace deal"
Umma Party spokeswoman
Garang, speaking to reporters, announced that he had achieved 50% success in his talks and said the two sides were to resume meetings to try to reach an agreement.
Meanwhile, Aljazeera's correspondent in Cairo also quoted a spokesman for the SPLA as saying that Garang would be back in the Sudenses capital Khartoum on 9 June, the first time after a 20-year absence.
"We think this peace deal is a positive step towards peace but it needs to be more encompassing of all political forces in Sudan, not just the two parties," al-Mahdi said.
Turabi engineered the coup in 1989 which overthrew Sadiq al-Mahdi's democratic government and placed President Omar Hassan al-Bashir in power. Turabi also jailed al-Mahdi. But Turabi is also married to Sadiq's sister, Wisal al-Mahdi.
The two parties are still not in total agreement. The Umma Party has said it will boycott the new government to be formed on 9 July, until elections are held in three years time.
Wait and see
But the PCP said it was waiting to see what the new constitution currently being formed will say.
Sudan's President Omar al-Bashir
has been in power since 1989
"We will wait to see whether the new constitution is truly representative and then we will decide whether to participate or not," Bashir Adam Rahman, head of the PCP's political bureau, told reporters.
Sudan's other main opposition party, the Democratic Unionist Party, did not sign the 16-party opposition alliance.