BBC News captures the essentials in its main article, which also includes maps and excellent video on both the South Sudanese declaration of independence and the story behind it:
South Sudan has become the world's newest nation, the climax of a process made possible by the 2005 peace deal that ended a long and bloody civil war. Sudan's President Omar al-Bashir and UN Secretary General Ban Ki-moon are among international dignitaries attending celebrations in the capital, Juba.
Sudan earlier became the first state to officially recognise its new neighbour. The south's independence follows decades of conflict with the north in which some 1.5 million people died.
Celebrations in Juba began at midnight (2100 GMT). A countdown clock in the city centre reached zero and the new national anthem was played on television.
South Sudan became the 193rd country recognised by the UN and the 54th UN member state in Africa.
Officials had planned for people to hold quiet celebrations at home, with the formal declaration of independence due later on Saturday. But the people clearly couldn't wait. Two hours before midnight and lines of cars zoomed around town packed with people waving flags and waiting to celebrate. When the final countdown arrived, the atmosphere was wild. Groups ran down roads, dancing to drum beats. Soldiers and policemen joined in too, waving paper flags and laughing. A sign read: "Congratulations, free at last, South Sudan." But the people didn't need to read the message - they were already dancing and leaping with happiness.
"It is a shout of freedom," said Alfred Tut, lifting his head back and screaming.
The BBC's Will Ross in Juba says the new country's problems are being put aside for the night, and there is an air of great jubilation. People are in the streets, cheering, waving South Sudan flags, banging drums and chanting the name of President Salva Kiir Mayardit, he adds.
A formal independence ceremony is due to be held later on Saturday. The Speaker of the South Sudan Legislative Assembly, James Wani Igga, is expected to read out the Proclamation of the Independence of South Sudan at 1145 (0845 GMT). Minutes later Sudan's national flag will be lowered and the new flag of South Sudan will be raised.
In addition to Mr Bashir and Mr Ban, attendees will include former US Secretary of State Colin Powell, the US permanent representative to the UN, Susan Rice, and the head of the US military's Africa Command, Gen Carter Ham.
Under the Comprehensive Peace Agreement, a referendum was held on independence, which was favoured by more than 99% of voters. The new country is rich in oil, but one of the least developed countries in the world, where one in seven children dies before the age of five. Unresolved disputes between the north and south, particularly over the new border, have also raised the possibility of renewed conflict.
On Friday, Sudan's Minister of Presidential Affairs, Bakri Hassan Saleh, announced that it recognised "the Republic of South Sudan as an independent state, according to the borders existing on 1 January 1956", when Sudan gained independence from Britain.
