By James Copnall
BBC News, Juba, southern Sudan
Former Prime Minister Sadiq al-Mahdi was among those at the conference
Opponents of Sudan's president have gathered in the southern capital, Juba, invited by the former rebels of the Sudan People's Liberation Movement.
Former enemies were smiling together as a historic conference opened in Juba.
But the meeting has been causing a political stir, with President Omar al-Bashir's party a notable absentee.
Semi-autonomous southern Sudan has been controlled by the SPLM since a 2005 peace deal ended more than two decades of civil war between north and south.
Northern politicians like the Islamist Hassan al-Turabi and the former Prime Minister Sadiq al-Mahdi once played major roles in successive Sudanese governments that fought the SPLM.
Now the two veteran politicians are in opposition, and in Juba at the SPLM's invitation.
Most of the leading Sudanese political parties have also shown up.
The notable absentee is the National Congress Party of President Bashir.
Over three days the delegates will discuss the key issues in Sudanese politics, including the faltering north-south peace process, and the ongoing civil war in Darfur.
Other topics include the controversial census, and the 2011 referendum in which the south will vote on whether it wants to remain part of Sudan.
The SPLM's Malik Agar has said the parties will not sign an alliance for next April's general elections.
But the gathering of so many political heavyweights in one place can only be of concern to those not there - and notably to President Bashir and his party.