The Sudanese government and the largest rebel group in the Darfur region have signed a peace deal aimed at ending three years of conflict.
The SLM's Minni Minnawi (r) did not sway the other two rebel groups
However, two smaller rebel groupings rejected it, despite last-minute efforts to secure their support.
The deal, which follows intense talks in Nigeria, calls for the disbandment of rebel forces and the disarmament of the pro-government Janjaweed militia.
The conflict has killed about 200,000 and left about two million homeless.
The peace plan, brokered by the African Union, creates a temporary regional government for Darfur, in which rebels will take part.
The Janjaweed are to be disbanded and the rebels incorporated into the security forces.
Deadlines came and went in recent days, as diplomats exerted pressure on parties after all the rebels had rejected the original draft.
In the end the Khartoum government and the Sudanese Liberation Movement (SLM) said they were willing to sign, despite reservations on both sides over power sharing and security.
"We are reaffirming that the fighting ends now in Darfur," SLM leader Minnawi said at Friday's signing ceremony in the Nigerian capital, Abuja.
"We shall go ahead with peace and we shall be serious."
US Deputy Secretary of State Robert Zoellick, who helped broker the deal, said it was a first step on what could still be a difficult path.
"This agreement, implemented by all of you, creates an opportunity for peace for your people," he said.
"There will be tests because not all have shown courage and leadership today."
The signing ceremony was delayed for three hours, as mediators tried to persuade the smaller faction of the SLM to sign up to the deal.
But in the end it would not budge, blaming a lack of trust in the security arrangements.
The smallest rebel group, the Justice and Equality Movement (Jem), called for fundamental changes to the document.
Its chief negotiator reiterated the rebels' demands for the post of vice-president in the Khartoum government and for Darfur to have a greater share of national wealth.
The BBC's Alex Last in Abuja says the groups that did not sign now face being isolated by the international community.
There is a danger, he adds, that the groups will eventually split, further destabilising a region that is already plagued by violence.
Aid organisations say the conflict has created one of the worst humanitarian crises in the world.
The rebels took up arms in 2003, accusing the government of discriminating against the black African residents of Darfur.
Pro-government Arab militia then launched a campaign, described as "genocide" by the US.
The Sudan government denies backing the Janjaweed militias accused of mass killing, rape and looting.