South Sudan's capital was rocked by heavy arms fire between forces loyal to the president and those of the vice president, causing widespread casualties and raising fears the country is returning to civil war.
The UN condemned the fighting in Juba and said it was ready to send more peacekeepers to South Sudan.
The fighting began on Sunday morning and continued until about 8pm local time, when a large thunderstorm seemed to put an end to the violence, said UN mission spokeswoman Shantal Persaud.
She confirmed that a UN armoured personnel carrier was hit by a shell at a camp to protect civilians. UN peacekeepers in the vehicle were wounded, said witnesses.
"The condition is really very bad. We have a lot of casualties this side, I think around 50 to 60 besides those of yesterday," said Budbud Chol who oversees security at a clinic in the camp.
"We have civilian casualties. We have rocket-propelled grenades that have landed in the camp which has wounded eight people."
Among the wounded are five children and two women while the rest were men, he said.
At least one person has died in the camp, he said, but he did not know about casualties outside where the fighting was heavy between government troops supporting President Salva Kiir and opposition forces loyal to Vice President Riek Machar.
The opposition side blamed government forces for starting the fighting with an attack on a rebel base in the Jebel area of the capital.
Three helicopter gunships bombed rebel camps, said William Gatjiath Deng, a spokesman for the rebel forces.
The UN Security Council held a three-hour emergency meeting on Sunday, and "condemned in the strongest terms" the escalation of fighting in Juba.
It expressed "particular shock and outrage at the attacks on UN compounds and protection of civilian sites. The council urged an immediate end to the fighting, stressing that "attacks against civilians and UN premises and personnel may constitute war crimes".
One Chinese officer was killed, and several Chinese and Rwandan peacekeepers were wounded in the attacks, said Japan's UN Ambassador Koro Bessho, who holds the rotating Security Council presidency.
Security Council members also expressed their readiness to consider enhancing the UN peacekeeping mission in South Sudan in an effort to prevent and respond to the violence.
About 10,000 Juba residents fled areas where there was fighting, said Jeremiah Young, policy adviser for World Vision in South Sudan.
"We have seen quite a few individuals packing up and leaving, trying to find shelter, what look like a lot of civilians taking off down the street, carrying their suitcases, their children," he said.
Other residents said they could not leave because of the fighting.
The fighting on Sunday appeared to be mainly in Jebel, where there is an opposition base and a UN base which houses thousands of internally displaced people, and Gudele, where the rebels have another opposition base.
Sunday's fighting was a resumption of the conflict after Friday, when more than 100 people died.
A precarious calm was restored on Saturday- the day South Sudan marked its fifth independence day - that was shattered on Sunday by the fighting.
South Sudan is trying to emerge from a two-year civil war caused by political rivalry between Mr Kiir and Mr Machar.
The two rival leaders issued a joint call for calm after Friday's fighting which began outside the presidential compound where they were meeting and soon spread through the city.
A similar skirmish in December 2013 sparked off the civil war that killed tens of thousands of people.