During yesterday's unrest a commercial plane was destroyed, and the local courthouse and the residence of the provincial governor William Powi were burnt.
Local authorities say no one was injured when a Air Niugini Dash-8 aircraft was set on fire and destroyed on the airstrip at Mendi airport. The situation in the town is tense, the provincial capital having gone into lockdown overnight.
According to police, it was yesterday's national court ruling upholding Mr Powi's 2017 election which sparked the rampage.
The election result had been challenged by losing candidates Joseph Kobol and Bernard Peter Kaku.
Prime Minister Peter O'Neill said a State of Emergency would be declared, with additional security forces to be deployed to prevent further unrest.
He said an emergency meeting of the National Security Council would take place this morning to approve emergency arrangements under appropriate laws.
"The actions we have seen today in Mendi are an absolute disgrace," Mr O'Neill said last night, adding that those involved would be detained by police and prosecuted.
"There is no place in in politics for this type of behaviour, and leaders involved with this activity will be held to account.
"All leaders must respect the rule of law, respect decisions of the court and respect the safety of people and property."