Some 22 suspected gang members were arbitrarily executed in a confrontation with police in western Mexico last year, Mexico's National Human Rights Commission said on Thursday, the latest grim example of rights abuses by security forces.

In May last year, federal police launched a raid against suspected members of the Jalisco New Generation Cartel (JNG) holed up in a ranch near the small town of Tanhuato in the violent western state of Michoacan.

At least 42 suspected gang members were killed and one police officer died in an incident that raised questions due to the one-sided death toll, one of the highest since President Enrique Pena Nieto took office in 2012, pledging to put an end to years of gangland violence.

"As a result of the investigation done by this organization, based on technical and scientific tests ... we established facts that imply grave human rights violations attributable to public servants of the federal police," Raul Gonzalez, the president of the CNDH, told a news conference.

Gonzalez said police lied about their role during the incident at the ranch, where they moved seven bodies and shifted weapons to manipulate the crime scene.

Police also tortured two people they arrested, and burned two bodies, Gonzalez added. The CNDH was unable to clarify how 15 of the victims were killed, he said.

The mother of a man who died on May 22, 2015, in a police raid lights a candle at the warehouse where her son was killed in a ranch in Tanhuato. An investigation by a human rights omission alleges police tortured some of the suspects that day and murdered others. (Frank Jack Daniel/Reuters)

The CNDH report is a fresh blow to Pena Nieto, whose approval rating has fallen to an all-time low over perceptions he has failed to tackle rampant crime and corruption.

Rights groups say that although Mexico's security forces face grave dangers in their fight against often brutal cartels, it is vital they hold themselves to higher standards.

"We knew this was what happened right from the start, we knew our sons were victims of abuse by the authorities," said Margarito Romero, father of one man who died that day. "They should have been arrested, not murdered ...even if some of them were members of the cartel, that is no excuse."

In a news conference, Renato Sales, Mexico's national security commissioner, did not accept police carried out executions. He said the investigation was continuing, and urged Congress to pass laws on when security forces can fire their weapons.

"In our view, the use of arms was necessary and proportional to the very real, imminent and lawless aggression," he said. "They acted in legitimate defence."

Lack of rules on use of force

Unlike the army, Mexican police do not have clear rules of engagement. Mexico's security forces — the federal police, the army and the navy, and others — have long been implicated in rights abuses during their decade-long battle with drug cartels that has claimed more than 100,000 lives.

"It's systematic and hopefully this will put a brake on the excesses and abuses by the federal police," said a senior Mexican law enforcement official who declined to be named. "This is very serious, and a massive blow to the government."

As recently as June, at least eight people died in confrontations between rebellious teachers and police in southern Mexico. Most notoriously, 43 trainees from the Ayotzinapa teaching college in south-western Mexico were apparently massacred in 2014 after being taken away by police.

Also in 2014, 22 suspected gang members were killed by army officers who were later accused of murder. The army argued they acted in self-defence and three soldiers were acquitted.

Police tape is seen at the entrance of the ranch in Tanhuato, state of Michoacan, Mexico, June 28, 2016. (Frank Jack Daniel/Reuters)

The Tanhuato incident last year pitted the federal police against suspected members of the JNG cartel, one of Mexico's newest and fastest-growing gangs.

In the weeks before the clash, the JNG gang had scored various major victories against security forces, including shooting down an army helicopter in the nearby state of Jalisco, killing six soldiers.

After the incident the government denied there had been any executions.

Earlier this year the Open Society Justice Initiative, a private human rights body, said incidents including Tanhuato constituted crimes against humanity.

It cited extrajudicial executions, official obstruction of investigations as well as a lack of justice, and said the International Criminal Court should step in if Mexico fails to resolve the cases.