Sudan's government and southern rebels have signed a permanent ceasefire as part of a peace deal to end one of Africa's longest-running civil wars.
The SPLM began fighting the Sudanese government in 1983
It was signed along with a detailed plan on how to implement earlier agreements just hours before the year-end deadline agreed with the UN.
Friday's ceremony in the Kenyan town of Naivasha is expected to lead to a comprehensive peace deal in January.
South African President Thabo Mbeki was there to witness the signing.
He was joined by his Sudanese counterpart, Omar al-Bashir.
The BBC's Ishbel Matheson, at the ceremony in Naivasha, said there were euphoric cheers as the documents were signed.
She says although there are many sceptics who say the plan is unworkable, the feeling in Naivasha is that the deal is done and peace is finally coming to south Sudan.
PRINCIPLES OF FINAL PEACE
Unify the two sides into 39,000-strong force if the south does not secede after six years
The south will have autonomy for six years followed by referendum for secession
To be shared 50:50
To be split 70:30 in favour of the government in the central administration
To be split 55:45 in favour of the government in Abyei, Blue Nile State and the Nuba mountains
Islamic law to remain in the north
Sharia in Khartoum to be decided by an elected assembly
"We now have all the components that will form the comprehensive Sudan peace agreement," said Kenyan mediator Gen Lazaro Sumbeiywo.
"Every topic on our agenda has been discussed and agreed on."
In the Sudanese capital, Khartoum, southerners took to the streets, singing and waving rebel flags, unhindered by police.
It was a rare display of southern sentiment in the tightly controlled capital.
The two sides had pledged in writing to the United Nations Security Council in November that they would sign a final peace by the end of the year, when a temporary ceasefire was due to expire.
The final January peace settlement will tie together agreements already signed by the government and the Sudan People's Liberation Movement (SPLM).
These deal with the share of power and wealth, the formation of a decentralised administration and steps towards a referendum on secession for the south in six years.
Some two million people have died since the war broke out in 1983, when rebels in the Christian and animist south demanded autonomy from the Muslim north.
The peace deal agreed for the south does not cover the unrest in Sudan's troubled Darfur region but it has been suggested it could become a template for a solution to that conflict.