US President Barack Obama has hailed the start of a landmark referendum on independence in Southern Sudan.
He said the week-long vote - which is expected to result in Africa's largest country being split in two - represented a "new chapter in history".
The poll was agreed as part of the 2005 deal that ended a two-decade civil war.
The mainly Muslim north has promised to allow the potential new country, where most people are Christian or animists, to secede peacefully.
Mr Obama said in a statement: "After 50 years of civil wars that have killed two million people and turned millions more into refugees this is the opportunity before the people of Southern Sudan."
He added that the action of Sudanese leaders would help determine whether Sudanese people move "toward peace and prosperity, or slide backward into bloodshed".
Sudanese President Omar al-Bashir has promised to respect the outcome, but warned an independent south would face instability.
Southern Sudanese voters are faced with two symbols on the ballot paper - a single hand for independence or two clasped hands to remain one country.
South Sudanese leader Salva Kiir, casting his ballot on Sunday, urged people to "be patient", in case they were not able to vote on the first day of polling.
Turnout in the referendum will be important, as the 2005 peace agreement stipulates that for the vote to be valid, 60% of the 3.8 million registered voters must take part.
Veronica De Keyes, head of the the European Union observer team in Juba, said voting appeared to have started well.
"What I observed this morning was very moving in the sense that you can feel it, in the crowd, the expectation of the people is important," she said on Sunday.
However the run-up to the vote was marred by clashes between the south Sudanese military and rebels in the oil-rich Unity state.
There are also reports of fighting between southerners and Arab nomads over grazing rights for their cattle in the disputed oil-rich region of Abyei, long seen as a potential flashpoint which could trigger wider violence.
Abyei was due to hold a separate referendum on whether to join north or south Sudan but this has been postponed indefinitely because of disagreements over eligibility.
North and south Sudan have suffered decades of conflict driven by religious and ethnic divides.
Southern Sudan is one of the least developed areas in the world and many of its people have have long complained of mistreatment at the hands of the Khartoum government.
Last week Mr Bashir said he understood why many southerners wanted independence, but he expressed concern at how the new nation would cope.
"The south suffers from many problems," told the Arabic news channel al-Jazeera.
"It's been at war since 1959. The south does not have the ability to provide for its citizens or create a state or authority."
Mr Bashir said southerners living in the north would not be allowed dual citizenship, and floated the idea of the two nations joining in an EU-style bloc.
The official result of the referendum is not due to be announced for at least four weeks, partly because of the logistical difficulties gathering the ballot papers from across a region the size of France and Germany that has few paved roads.