Sudan's Vice-President John Garang, a former rebel leader, has been killed in a crash, the government has said.
Garang was hailed a peacemaker just three weeks ago
Mr Garang had been missing since Saturday, when contact was lost with his helicopter flying back from Uganda.
The BBC's Jonah Fisher says Mr Garang's importance in holding together southern Sudan cannot be overstated.
He was greeted as a peacemaker by more than one million people when he was sworn in three weeks ago as part of a deal ending a 21-year civil war.
His death will be a huge blow to the Sudanese people, our correspondent adds.
Three days of national mourning have been declared.
Mr Garang was returning to his base in New Site, southern Sudan after meeting Ugandan President Yoweri Museveni at his ranch in Rwakitura, about 300 km (190 miles) south-west of the capital, Kampala.
Six of his associates and the seven-member crew of the Mi 72 Ugandan presidential helicopter also died in the crash, which is being blamed on bad weather.
Mr Garang's deputy in the Sudan People's Liberation Movement, Salva Kiir Mayardit, will take control of the former rebel group, the SPLM said.
Mr Mayardit said the SPLM remained committed to the peace agreement Mr Garang signed in January and urged all Sudanese to remain calm.
Sudan's President Omar al-Bashir said he was confident that the peace agreement would remain on course.
The mediator in the long-running Sudanese peace talks, Kenyan General Lazaro Sumbeiywo, said it was clear from the peace agreement that whoever became SPLM leader would also take over as Sudan's vice-president.
Mr Garang's widow, Rebecca, told the BBC that the people of Sudan should carry on with his vision.
Mr Garang steered the mainly Christian and animist SPLM through a bloody war against the Islamic government in the north, which left at least 1.5 million people dead.
He ruled the group with an iron hand, imprisoning or killing anyone who threatened to stand in his way.
But he managed to keep the disparate movement together, despite many disagreements.
The conflict in Sudan ended with the signing of a peace agreement in January and Mr Garang became vice-president in a new government of national unity.
He declared the peace agreement a "great battle and a major victory".
The dignity of the southern people, he said, had been restored: "Nobody will take us for granted - we have come to stay".
With his death the future of peace in Sudan is once more in the balance, correspondents say.