Violence often damages the remote hinterland of the resource-rich Pacific country, where tribal and land disputes override regional politics.
The armed crowd was angry after the court rejected the election of the regional governor. They set fire to an airplane, looted a warehouse and set fire to a building in Mendi, capital of Southern Highlands province.
Prime Minister Peter O'Neill said Papua New Guinea had declared a nine-month emergency in the Southern Highlands Province, and temporarily suspended its government.
"The actions of the person who damaged the property in Mendi have shocked the nation," O'Neill said on his website as quoted by Reuters on Saturday (16/06/2018).
"The police will investigate every agitator, and everyone involved in the riots," he added.
Thomas Eluh, a former police officer and deputy administrator in the province, has been granted constitutional powers.
"Police, including mobile squads, are immediately deployed, along with criminal investigators," O'Neill added.
More than 200 troops from Papua New Guinea Defense Forces will be flown to the city of Mount Hagen on Saturday before traveling to Mendi, PNG's Loop Loop media said.
Reuters could not independently confirm reports of troop movements.
The looters searched the earthquake relief supply shed during the Mendi riots, said Barclay Tenza, a provincial disaster spokesman.
"They took all the food," he said over the phone from Port Moresby.
Many communities are still receiving aid after a magnitude 7.5 earthquake in February that killed 100 people, testing the financial and capacity of one of the world's poorest countries.