Thousands of jubilant south Sudanese queued through the night to be among the first the vote on Sunday in the region's landmark independence referendum, AFP correspondents reported.
At polling stations across the regional capital Juba, hundreds of voters were already waiting well before dawn to seize their opportunity to have their say on whether the impoverished south should break way from rule by Khartoum after five decades of devastating conflict.
"We are standing in the queue to step forward to independence," said David Akol, as he waited with hundreds of others to vote at the memorial to veteran rebel leader John Garang, who died shortly after signing the 2005 peace deal with the Khartoum government that provided for Sunday's vote.
"The day that we have waited for for so long has finally arrived."
Yar Mayon, who grew up in refugee camps in Ethiopia and Kenya, said: "I came here in the early morning because I wanted to show just how much I wanted to vote.
"It was so important to me I could not sleep," she said, as she prepared to cast her ballot in favour of partitioning Africa's largest nation and creating the world's 193rd UN member state.
As the sun rose shortly after 7:00am (0400 GMT), Wilson Santino said: "This is a new dawn because we vote for our freedom.
"We have been fighting for too many years but today this vote for separation is also for peace. Soon the sun will be shining over a free south Sudan."
Polls were to open at 8:00am (0500 GMT) and close at 5:00pm (1400 GMT) for the first of seven days of voting.
Euphoria had gripped Juba on the eve 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.
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.
"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.
In an opinion article published by the New York Times on Saturday, President Barack Obama said voters must be allowed to make their choice free from intimidation and coercion.
"Now, the world is watching, united in its determination to make sure that all parties in Sudan live up to their obligations," Obama wrote.
"All sides should refrain from inflammatory rhetoric or provocative actions that could raise tensions or prevent voters from expressing their will."
Obama said that if the Khartoum government lived up to its obligations under the 2005 peace deal and respected the outcome of the vote, it could be removed from a US list of state sponsors of terrorism.
"Today, I am repeating my offer to Sudan's leaders -- if you fulfil your obligations and choose peace, there is a path to normal relations with the United States," he wrote.
Aides say that Obama, grounded in a commitment to Africa, from where he traces part of his lineage, left administration officials in no doubt of the grave stakes posed by Sudan's potential split.
"'Let me be clear about what this means to me,'" one official quoted Obama as saying during a staff meeting earlier this year.
"'Two million people died the last time there was a conflict between north and south. That cannot happen again.'"
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."