South Sudanese celebrated the birth of their nation on Saturday after voting for independence in a referendum under the terms of a 2005 peace deal that ended decades of war.
The new state has its capital in Juba and was officially recognized on Friday by the government of Sudan, based in Khartoum, hours before the formal split took place.
Officials said the birth of the new nation would take place at midnight between July 8 and 9, and a formal independence ceremony was due to be held later on Saturday.
"At midnight, bells will be rung across the new country, and drums will be sounded, to mark the historic transition from southern Sudan to the Republic of South Sudan," a statement from the southern government said.
According to the official program, a formal Proclamation of the Independence of South Sudan will be read out by southern parliament speaker James Wani Igga at 11:45 a.m. (0845 GMT). Minutes later Sudan's national flag will be lowered and the new flag of South Sudan will be raised.
The underdeveloped but oil-rich new republic won its independence in the climax of a 2005 peace deal that ended decades of civil war with the north.
The Khartoum government was the first to recognize the new state, hours before the formal split took place, a move that smoothed the way to the division of what was, until Saturday, Africa's largest country.
But recognition did not dispel fears of future tensions.
Northern and southern leaders have still not agreed on a list of sensitive issues, led by the exact line of the border and how they will share oil revenues, the lifeblood of both economies.
In Juba, people on the corners of dirt streets waved flags and danced in the lights of car headlights, chanting "SPLM o-yei, South Sudan o-yei, freedom o-yei" just before midnight.
The Sudan People's Liberation Movement (SPLM) led the rebel movement that fought the north until 2005 and now dominates the southern parliament.
In Khartoum, just before the split, Sudanese President Omar Hassan al-Bashir, who now leads just the north, told journalists he would attend the independence celebrations in the south later on Saturday.
"I would like to stress ... our readiness to work with our southern brothers and help them set up their state so that, God Willing, this state will be stable and develop. The cooperation between us will be excellent, particularly when it comes to marking and preserving the border so there is a movement of citizens and goods via this border," Bashir said.
Analysts have long feared a return to civil war if disputes are not resolved.
After the stroke of midnight the Republic of Sudan lost around three quarters of its oil reserves, which are sited in the south, and faced the future with insurgencies in its Darfur and Southern Kordofan regions.
UN Secretary General Ban Ki-moon told reporters in Juba on Friday evening he was confident South Sudan would soon join the global body.
Earlier in Khartoum, he urged the northern government to allow UN peacekeepers to stay past their mandate to monitor the situation in Southern Kordofan, the north's biggest remaining oil state, and other hotspots.