Mexican police arbitrarily executed nearly two dozen suspected gang members on a ranch last year, the government's National Human Rights Commission said on Thursday, one of the worst abuses by security forces in a decade of grisly drug violence.

In May last year, federal police ambushed suspected members of the Jalisco New Generation Cartel (JNG) holed up at Rancho El Sol near the small town of Tanhuato in the violent western state of Michoacan and killed 42 men.

Only one policeman died in the fight, in which police backed by a Black Hawk helicopter attacked the cartel, a kill rate way higher than international norms, but not uncommon in Mexico's drug war. Only one injury was reported.

The one-sided toll was one of the highest since President Enrique Pena Nieto took office in 2012, pledging to end years violence.

"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, moved 7 bodies and shifted weapons to manipulate the scene. Police tortured two people they arrested, and burned two bodies, Gonzalez added.

The CNDH was unable to clarify how another 15 of the victims were killed, he said.

The report is a fresh blow to Pena Nieto, whose approval rating is at an all-time low over perceptions he has not tackled rampant crime and corruption.

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

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

It said the International Criminal Court should step in if Mexico fails to resolve such cases.

"They should have been arrested, not murdered ...even if some of them were members of the cartel, that is no excuse." said Margarito Romero, father of one man who died that day.

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 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."

Mexico's federal police, the army and the navy have long been implicated in abuses since a drug war that has claimed more than 100,000 lives began in 2006.

"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."

Police killed 17 people for every officer lost in gunbattles in 2014, according to a study by Mexico's National Autonomous University, a number the study said was consistent with excessive force.

And in shootouts involving Mexican police between 2007 and 2013, the number of people killed for each person injured rocketed from 1.6 to more than 20, the study said.

In 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 police snatched them.

The same year, soldiers killed 22 suspected gang members. The army argued they acted in self-defense and three soldiers were acquitted of murder charges.

CHAOTIC AFTERMATH

Almost all of the men who were killed in Tanhuato, more than 30 of them, came from the town of Ocotlan across the state border in Jalisco and less than an hour's drive from the sprawling 112-hectare (277-acre) alfalfa farm the JNG cartel had recently seized.

Photos released within hours of the early-morning massacre showed partially dressed men face down in fields surrounding the main house. Family members and forensic experts believe some were gunned down as they tried to escape.

Most appear to have been sleeping on a verandah and then scattered when the police showed up.

On a recent visit, Reuters witnesses saw the tin roof of a large barn, where several of the men died, riddled with hundreds of bullet holes, possibly fired from the helicopter which TV images that day showed hovering over the building.

The abandoned ranch house was pocked by high caliber ammunition, some penetrating thick walls. Shattered glass surrounded a stagnant swimming pool.

In the weeks before, the cartel had hit security forces hard, including shooting down an army helicopter, killing six soldiers.