"Myanmar is plumbing the depths of the human rights abyss," said James Rodehaver, head of the United Nations rights office's Myanmar team.
Speaking to reporters in Geneva, he said the office's latest report on the situation in the war-torn country had found "massive regressions in human rights that have been provoked by a vacuum of rule of law".
"Myanmar military has created the crisis by instrumentalising the legal system, criminalising nearly all forms of dissent against its attempts to rule the country," he said.
Myanmar has been in turmoil since the military ousted Aung San Suu Kyi's government and seized power in 2021.
The junta is struggling to crush resistance to its rule by long-established ethnic rebel groups and newer pro-democracy forces.
Since the coup, at least 5,350 civilians have been killed and more than 3.3 million displaced, while nearly 27,4000 people have been arrested, Tuesday's report said.
"Credible sources indicate that at least 1,853 people have died in custody, including 88 children and 125 women," rights office spokeswoman Liz Throssell told reporters.
Rodehaver said that since the rights office's last report on Myanmar was published 15 months ago, the mass arrests had continued and the rights situation had continued to deteriorate.
He voiced particular concern over the "horrific conditions" in detention facilities, where torture and ill-treatment was "pervasive".
"Detainees interviewed by our office describe methods such as being suspended from the ceiling without food or water, being forced to kneel or crawl on hard or sharp objects, the introduction of animals such as snakes or insects or other wild animals in order to provoke fear and terror in individuals," he said.
Others, he said, described beatings with iron poles, bamboo sticks, batons, rifle butts, leather strips, electric wires and motorcycle chains.
There were also accounts of "asphyxiation, mock executions, electrocution and burning with tasers, lighters, cigarettes and boiling water".
"There are truly some of the most depraved behaviours utilised as methods of torture in these detention centres," Rodehaver said, also decrying "extremely disturbing reports ... of sexual violence both against male and female detainees".
The UN rights office stressed the urgent need for those behind the vast array of abuses to be brought to justice.
"The lack of any form of accountability for perpetrators is an enabler for the repetition of violations, abuses and crimes," Throssell warned.
UN rights chief Volker Turk was once again calling on the UN Security Council to refer the situation in Myanmar to the International Criminal Court, she said.