Fireworks lit up the sky and packed cars drove around the city with drivers honking their horns and passengers waving their new country's flag from the windows.
The noise from the large crowd gathered around countdown clock, at the main crossroads in the new country's capital, was deafening, an AFP reporter said.
Standing next to the flashing clock, which read "free at last," 27-year-old university student Andrew Nuer could barely describe how he felt as cars hooted around him.
"We have struggled for so many years and this is our day -- you cannot imagine how good it feels," said Nuer, who had just come back from Cairo to celebrate independence.
"We pray to God in the future to help us make this a prosperous and peaceful country, and to show the world that we can do it," he added.
Hours earlier, world leaders including UN chief Ban Ki-moon had flown into Juba for Saturday's official celebrations.
"Fifty years fighting for independence and if this is freedom, then this is great," said Daniel Bol, banging his tin drum.
One sign on the back of a car full of flag waving southerners read: "Just divorced."
South Sudan's independence comes exactly six months after a referendum that saw southerners vote almost unanimously to split with their former civil war enemies in the north.
For decades, until a peace agreement was signed in 2005, southern rebels fought two wars with successive northern governments for greater autonomy and recognition, a conflict that left the region in ruins and millions of people dead.
"We are free! We are free! Goodbye north, hello happiness!" screamed Mary Okach.
Copyright AFP (Agence France-Presse), 2011