Shops and cars were set on fire and supermarkets looted after police went on strike over a pay dispute.
The absence of police on Wednesday encouraged people from the city's outskirts to ransack shops and cause wider destruction, locals told the BBC.
It follows wider tensions in Papua New Guinea over rising costs and high unemployment.
Prime Minister James Marape addressed the nation on Thursday apologising for the incident, but saying lawlessness would not be tolerated.
"Breaking the law does not achieve certain outcomes," he told the public.
While most of the violence had been curbed by Wednesday evening, after soldiers were deployed and police resumed duty, Mr Marape acknowledged that the situation was "still tense".
The Port Moresby General Hospital had confirmed eight deaths in the capital, while another seven people were reported to have died in the city of Lae, Papua New Guinea's second largest.
"We have seen unprecedented level of strife in our city, something that has never happened before in the history of our city and our country," National Capital District Governor Powes Parkop said in a radio address on Wednesday, according to a Reuters report.
He said the looting had largely been carried out by "opportunists". Some violence was also recorded to have been carried out by police demonstrators.
Shops and cars were set on fire and supermarkets looted across the city
The unrest was triggered after police and other public servants staged a protest strike outside parliament on Wednesday, after discovering their pay had been reduced by up to 50% in their latest pay-check.
In response, Prime Minister James Marape said the pay cut was an error due to a computer glitch - which had deducted up to $100 (£78) from the pay-checks of public servants. He said the administrative error would be corrected at next month's payments.
But this answer was not accepted by many protesters, some of whom then tried to push into the parliament - with footage showing people torching a car outside the Prime Minister's compound and overrunning a gate.
Many touted theories from social media that the government was raising income taxes- an assertion denied by the government.
"Social media picked up on this wrong information, misinformation," said Mr Marape, according to the New York Times, adding that people had taken advantage of police being off the streets.
Port Moresby local, Maholopa Laveil, told the BBC opportunists ransacked the city, setting many buildings and small shopping centres on fire and stealing cars. The worst of the violence happened during the day.
"We had a lot of fear for people who were working in the shopping centres and offices - there was a lot of glee and excitement from the people who were attacking and entering the shops," he said.
Mr Laveil, who is an economic lecturer at the University of Papua New Guinea, also said most of the people looting appeared to be from the poorer settlements outside the city.
"They had come on when they heard the police had stood down and were not policing the city.
"These are suburbs with really poor people, who don't have jobs and who contribute to a lot of crime and lawlessness in the city. Many have suffered a lot from not being employed- inflation pressures - and they came out in numbers trying to get what they could from the shops nearest to them," he said.
Ambulance officials said they had attended to several shooting injuries, while the US embassy reported shots near its compound.
The Chinese embassy has also lodged a complaint with the PNG government, saying several Chinese businesses were attacked and a number of Chinese nationals injured - though they did not specify how many.
"The Chinese Embassy in Papua New Guinea has lodged solemn representations with the Papua New Guinea side over the attacks on the Chinese shops," the embassy said on WeChat.
People breaking into shops in Port Moresby
Australia, a neighbouring and major security partner for PNG, on Thursday called for calm in the country.
Mr Marape, who met with Australia's leader last month, has yet to ask for peacekeeping help from the country.
Amid an economic slump in his country that has seen higher inflation and unemployment rates, the prime minister has faced increased pressure and public resentment from many groups in society.
The political opposition has also been working on a motion to cast a vote of no confidence in the Prime Minister, scheduled for February.
"Households are doing it tough, there's growing inequality within urban centres particularly the capital city and we've got a growing number of settlements and high crime rates," Mr Laveil said.
"All of those factors contributed to a perfect storm."