Nearly 150 people were killed after heavy gunfire broke out Friday between soldiers loyal to President Salva Kiir and others backing Vice President Riek Machar, according to Machar's spokesman, James Gatdet Dak.
CNN could not independently verify that number.
"South Sudan today marked the most horrifying Independence Day in the world this year," Dak wrote in a post on Facebook.
He said "sporadic shootings continued throughout the night in pockets of areas around Jebel and Gurei" but relative calm had returned to the capital, Juba, on Saturday.
The latest violence apparently started as Kiir and Machar were meeting to discuss previous clashes between their forces. Outside the presidential compound where the meeting took place, a gun battle kicked off, although it's unclear between whom.
'Plot to target civilians'
Nyargi Roman, another spokesman for Machar, said that many presidential guards who had been stationed outside the presidential compound were missing and feared dead. Local news reported that 146 civilians and soldiers were killed.
On Saturday, security checkpoints appeared across the city, and cellphone networks were not working properly. The Army Chief of Staff urged civilians to stay indoors. There were reports of troop movements across the country, but it was unclear if it was related to the fighting.
The spokesman for the Office of the South Sudan Chief of General Staff said in a statement read on state television that authorities had "credible information of a plot to target civilians in Juba and adjacent areas."
Forces will "act accordingly to protect the lives of civilians at any cost," said the spokesman, Brig. Gen. Lul Ruai Koang.
Echoes civil war
South Sudan gained independence in 2011 after 98% of the population voted to break away from Sudan. The East African nation, the youngest country in the world, quickly fell into civil war that took on ethnic undertones.
The violence on Friday echoes the beginning of South Sudan's civil war. In December 2013, soldiers from Kiir's Dinka ethnic group tried to disarm Nuer soldiers perceived to be loyal to Machar. Soldiers targeted Nuer civilians in the ensuring fighting, Human Rights Watch says.
The ensuing civil war was gruesome -- at least 50,000 were killed, more than 2 million displaced, and nearly 5 million people faced severe food shortages. Under a peace deal signed in August, Kiir is the president of the country and Machar is the first vice president, but it hasn't stopped fighting.
Kiir and Machar held a joint news conference after their meeting was cut short.
"What is happening outside is something we cannot explain to you," Kiir told journalists.
"This incident will be controlled and measures will be taken so that peace is restored," Machar said.
The fighting quickly spread from the presidential compound across Juba.
Command and control breakdown
"We heard heavy artillery fire at the U.N. (base), and that continued for about an hour or so and then stopped. It was coming form the outer perimeters of the compound," said Shantal Persaud, acting spokeswoman for the United Nations Mission in South Sudan.
Helicopter gunships were seen in the sky, and tanks rumbled through the streets. Under the peace deal, both government and opposition troops were stationed in Juba, a plan which many criticized because it put both forces in close proximity. On Friday night, it was unclear if either Machar or Kiir were in full control of their forces.
"What we may be seeing is a total breakdown of command and control in Juba," said Kate Almquist Knopf, director of the Africa Center for Strategic Studies. "We need to watch carefully for whether a cycle of reprisal killings by either side begins in the next few days."
U.N. Secretary-General Ban Ki-moon said the fighting was "a new betrayal of the people of South Sudan, who have suffered from unfathomable atrocities since December 2013"
Both Kiir and Machar are accused of war crimes.
Two weeks ago, fighting in the western city of Wau between government and opposition troops displaced at least 70,000, according to the U.N. The country is nearly out of money because it comes almost exclusively from oil revenue -- the value of which has plummeted in recent years. People have become desperate. In lieu of payment, government soldiers have reportedly been allowed to rape women, a U.N. report said.
"I urge President Kiir and First Vice-President Riek Machar to put an immediate end to the ongoing fighting, discipline the military leaders responsible for the violence and finally work together as partners," Ban said.