One of Africa’s best-known political figures, Garang was killed in a helicopter crash near Uganda’s border with Sudan on Saturday. He was trying to get back to his base in southern Sudan from a meeting with Uganda’s President Yoweri Museveni.
The MI-72 helicopter belonging to the Ugandan government came down, apparently because of a lack of fuel, in bad weather. All of the 14 Ugandans and Sudanese reported to have been on board, were killed.
John Prendergast, said Garang’s death could also expose serious internal "schisms" within the Sudan People’s Liberation Movement (SPLM), the rebel group Garang led for 22 years.
Prendergast is a special advisor to the International Crisis Group (ICG), a global think-tank.
"Within the SPLM/A, there is a long history of divisions, and unless they can thrash out their differences quickly, it [Garang’s death] has the potential to create more turmoil," he told IRIN by phone from the Ugandan capital, Kampala.
"Some people in Khartoum, lone-rangers and spoilers who always felt left out, will try to exploit the situation," he added.
The SPLM and its army sealed a peace agreement with the Sudanese government on 9 January 2005. As part of the agreement, Garang became the country’s First Vice-President. He was sworn in on 8 July.
Prendergast told IRIN Garang's death was a setback to that agreement. "Garang was so central to the negotiations, and many of the provisions were his," he said.
The Comprehensive Peace Agreement was signed in Kenya after several years of talks mediated by Lt Gen Lazarus Sumbeiywo, a retired Kenyan army officer. Sumbeiywo is reported to have described Garang’s death, as "shocking, the loss of a visionary leader".
"My prayer is that the Sudanese will remain level-headed," he was quoted as saying.
Sudan’s President, Umar el-Bashir, has said he will work with the SPLM to continue to implement the peace agreement.
"We are confident that the peace agreement will proceed," Bashir said on television in the Sudanese capital of Khartoum.
Despite this, a number of people were reported killed as violence erupted in Khartoum after news of Garang’s death. Military personnel were deployed on the city’s streets, where gangs of youths roamed. An oil depot and vehicles were said to have been set alight.
Humanitarian sources said disturbances were reported in the southern cities of Juba and Malakal.
From Nairobi, Garang’s Chief-of-Staff, Commander Salva Kiir Mayardit, also called for calm.
"We want to assure everyone that the leadership and the cadres of the SPLM/SPLA will remain united and strive to faithfully implement the CPA [peace agreement]," he said in a statement.
According the agreement, the former rebel group has two weeks to name a successor. Salva Kiir, a man held to have a more secessionist agenda for the south of Sudan than Garang did, is thought a likely candidate for Garang’s posts, not just as First Vice-President, but also as President of southern Sudan for a six-year interim period.
Kiir said an emergency meeting was to be held at New Site, the base Garang was trying to reach in southern Sudan when he died.
At Garang’s home in the Kenyan capital, Nairobi, hundreds of devastated southern Sudanese gathered to give their condolences to people at his house. A family source told IRIN a memorial service would be held at New Site in the next few days.
In a separate statement, retired Kenyan President Daniel Arap Moi, said he had recently spoken to Garang. "He told me about his aims at consolidating peace in the whole of the Sudan, and the reconstruction of southern Sudan," Moi said. "His passing will bring a lot of confusion."
Garang had always maintained that the south of Sudan should remain politically joined to the north. In the south, his policy met with popular opposition, but it found support among northern rebel movements still fighting the Khartoum government.
From Eritrea, Salah Barqueen, a member of a rebel group known as the Beja Congress which has recently assaulted Sudan military outposts, described Garang’s death as "a big sorrow - not just for the southerners, but for all Sudanese people who are seeking a justified and comprehensive peace."
The Sudanese government on Monday declared three days of mourning for Garang’s death