A drunk police officer has allegedly shot and killed three teenagers at a party in Timor Leste, prompting protests against police brutality and lack of discipline.

The suspected officer, Jose Mina, was off duty at a memorial celebration in the Kuluhun district of Dili on Saturday, when he allegedly drew his firearm in an attempt to stop two people arguing, Associated Press reported.

According to the report, the electricity went out and Mina fired indiscriminately into the dark, killing three 18-year-old men and injured five others.

East Timor police chief Julio da Costa Hornay said Mina had been arrested and police were investigating the shooting, which had “damaged the reputation of the Timor Leste police institution”.

Hornay said officers who carried their weapons off-duty had “violated the law and had to take the consequences”.

Video posted to social media showed at least two of the dead men covered in blood and being carried by friends at the hospital.

“The government and police should clarify why police officers can use weapons freely to kill people,” Alberto Sequeira, the father of one of the victims, told a news conference.

Sequeira said it was not the first time.

“Armed policemen have threatened other families repeatedly but the government has not taken serious measures and now civilians become victims.”

Across Dili, and particularly in the Kuluhun neighbourhood, walls were scrawled with anti-police graffiti in response to the killings, and there were reports of unrest.

“In the past, our fathers were killed by Indonesian guns. Now, our children are killed by PNTL [police force] arms,” read one wall.

A demonstration against police brutality in the Timor Leste capital of Dili. Photograph: Bardia Rahmani/ Fundasaun Mahein/The Guardian

Students who had planned an anti-government corruption rally in the city instead spoke out against the police, many of whom were standing nearby.

The Timorese security watchdog, Fundasau Mahein, called for structural reform of the police force.

“The Kulu-Hun incident highlights the need for an overhaul of PNTL; it is not enough to simply argue that these problems are the result of a few bad apples within the police force,” it said.

“While [Fundasaun Mahein] strongly believes that the PNTL member responsible for this incident should be subject to criminal sanctions, it also insists that PNTL as an institution take responsibility.”

Bardia Rahmani, Fundasaun Mahein’s international adviser, told Guardian Australia the Timor Leste police force had long-running issues with discipline and abuses of power.

Rahmani listed a number of recent incidents, including teargassing and beating student protesters, and a non-lethal shooting between police and military at a music festival, which she said were part of a “culture of impunity” in the force.

“There have been numerous cases of police shooting young men, resulting in injury or death, including in the districts of Ainaro in 2017, Covalima in 2016, and Hera in 2012. Recently, there was a case of police brutality in which an off-duty police officer detained, beat and teargassed a man with whom he had had a traffic dispute.”

Rahmani said there had been improvements, including a community policing model which many residents supported, but it had not been rolled out across the force.

“Police training too often emphasises an adversarial, authoritarian relationship with the public, and police officers frequently carry their guns while off-duty and use their positions to resolve personal disputes,” he said. “The accumulation of incidents like this has led to anger among the public toward the police.”