After 50 years of devastating conflict, south Sudanese were finally to get a vote on whether to remain part of Africa's largest nation or break away as the world's 193rd state.
Polls were to open at 0500 GMT on Sunday and close at 1400 GMT for the first of seven days of voting in a historic referendum that was the centerpiece of a 2005 north-south peace deal that ended Africa's longest-running conflict.
Euphoria gripped the regional capital Juba on the eve of the launch of polling as people feted the looming end of a long and often difficult countdown.
Hollywood star George Clooney joined a host of current and former world statesmen including senior US Senator John Kerry, former president Jimmy Carter and ex-UN chief Kofi Annan in the city for south Sudan's big day.
But the celebrations were overshadowed by deadly clashes with armed tribesmen and renegade militiamen in two remote oil-producing districts on the north-south border that were bitterly contested in the 1983-2005 civil war.
US President Barack Obama said Saturday voters must be allowed to make their choice free from intimidation and coercion.
Obama also said in an opinion article for the New York Times that if it lived up to its obligations under a 2005 peace agreement, the Khartoum government could be removed from a US list of state sponsors of terrorism.
"Now, the world is watching, united in its determination to make sure that all parties in Sudan live up to their obligations," Obama wrote in the Times.
"As the referendum proceeds, voters must be allowed access to polling stations; they must be able to cast their ballots free from intimidation and coercion.
"All sides should refrain from inflammatory rhetoric or provocative actions that could raise tensions or prevent voters from expressing their will."
The US leader, who has orchestrated an intense diplomatic effort to ensure the referendum goes ahead on time, and without violence, said all sides must resist prejudging the outcome of the referendum as the ballot is counted.
"In the days ahead, leaders from north and south will need to work together to prevent violence and ensure that isolated incidents do not spiral into wider instability.
"Under no circumstance should any side use proxy forces in an effort to gain an advantage while we wait for the final results," Obama said ahead of a referendum following a 50 year civil war which killed two million people.
South Sudanese president Salva Kiir told his people in an eve of polling day message that there was no alternative to peaceful coexistence with the north.
"Fellow compatriots, we are left only with a few hours to make the most vital and extremely important decision of our lifetime," he said.
"Today there is no return to war. There is no substitute for peaceful coexistence.
"The referendum is not the end of the journey but rather the beginning of a new one," he added, alluding to the six-month transitional period to recognition as an independent state stipulated by the 2005 peace agreement.
US envoys had led an intensive international diplomatic effort right up to the last minute to ensure that the referendum went ahead as scheduled under the deal. Washington's Sudan envoy Scott Gration alone made 24 trips to the region.
The conflict between the Muslim, mainly Arab north, and the African, mainly Christian south, has blighted Sudan virtually since independence from Britain in 1956, fuelled by religion, ethnicity, ideology and resources, particularly oil.
President Omar al-Bashir, an army man who led the north's war effort against the south for a decade and a half before signing the 2005 peace deal, has said he will respect the outcome of the vote if it is "free and transparent."
On Thursday, the UN Security Council said its members "appreciate" that stance, in a rare act of praise for a man who is wanted by the International Criminal Court for war crimes, crimes against humanity and genocide over his government's handling of the seven-year-old rebellion in Darfur.
The outcome of the referendum will be decided by a simple majority, but the peace deal stipulates that 60 percent of the nearly four million voters must turn out for the result to be valid.
Ballot papers carry symbols as well as the referendum question in both English and Arabic -- an estimated 80 percent of adults cannot read or write after entire generations missed out on an education during the civil war.
If south Sudan does become independent, it will rank as one of the poorest nations in the world and remain heavily dependent on international aid.