Mexico's police covered up the rape and abuse of demonstrators at a land rights protest, Amnesty International has said
Prosecutors destroyed witness statements and prison doctors refused to verify claims of abuse, according to testimony taken by the rights group.
A local human rights group recorded 23 complaints of sexual abuse after the protests in San Salvador Atenco.
The State of Mexico governor has in the past dismissed similar allegations.
Governor Enrique Pena has repeatedly rejected calls for an independent inquiry into what happened in May, and has described abuse claims as a tactic used by "subversive groups" to undermine the authorities.
A spokeswoman for Amnesty described the incidents as "acts of torture".
"The state authorities have placed the burden of proof on the victims, while seeking to discredit their allegations," said Kerrie Howard, deputy director of the group's Americas programme.
The clashes began on 3 May after protesters armed with machetes and sticks captured 12 police officers who had been trying to break up a demonstration by flower sellers.
A total of 212 people were arrested in two days of clashes in San Salvador Atenco and nearby Texcoco, a third of whom have since been released without charge.
Rupert Knox, a researcher with Amnesty, told the BBC News website there was a question mark over whether the attacks on protesters were isolated incidents or part of a planned operation.
"The police were very hyped up and seemed to perceive the whole population as subversives who had murdered police," he said.
"People were dragged from their beds and kicked and beaten relentlessly before being bundled into police vehicles, at which time the women also suffered sexual abuse."
The report said the worst of the alleged abuse occurred while the protesters were being taken to prison in trucks.
"The police forced them to remain piled on top of each other on the floor of the vehicles for several hours so that they felt as though they were going to suffocate while at the same time being trampled on by the police officers," the report stated.
"People held in several of the vehicles were repeatedly subjected to beatings, insults and death threats. Several women reported that they were subjected to sexual violence on the journey."
An unnamed woman who gave testimony to Amnesty described how she was abused in one of the police trucks.
"They pushed me to the ground and also kicked and punched me as well as hitting me with sticks and truncheons," she said.
"They pulled my hair and stamped on me and then picked me up and pushed me face down into a police van, all the while beating me ... amid all the blows, insults and pushing, they started to grope me."
Another said that an official had ripped her statement from a typewriter when she started detailing a complaint of rape.
A third said that a prison doctor had refused to give her a medical examination to verify her rape claims, saying she risked losing her job. The doctor instead offered her pessaries and a painkiller, according to the woman's testimony.
The report said 30 of about 2,000 officers who took part in the operation were facing action for use of violence and abuse of authority.
The towns where the demonstrations took place, to the north-east of Mexico City, saw protests in 2002 when locals forced the government to back down from plans to build a new airport for the capital on expropriated land.
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