Venezuela "brutally" abused opposition activists between April and September this year, rights groups claimed, during a heavy-handed crackdown on anti-government protests that led to scores of deaths on all sides.

Government security forces tortured some of thousands of detainees with "electric shocks, asphyxiation, sexual assault, and other brutal techniques," according to a joint report published by Human Rights Watch (HRW) and Penal Forum (PF) on Wednesday.

In one case, a 34-year-old man described how he was hung from the ceiling by plainclothes police and shocked with a metal rod to extract a confession. In another, a 17-year-old boy said he was beaten with sticks and held in an overcrowded punishment cell for days.

Jose Miguel Vivanco, Americas director at HRW, said impunity for the perpetrators of violence suggested "government responsibility at the highest levels."

"These are not isolated abuses or occasional excesses by rogue officers but rather a systematic practice by Venezuelan security forces,” he said.

Read more: UN slams Venezuela for using excessive force, arrests to crush protests

A political weapon

HRW and PF documented 88 cases of rights violations — including excessive force and arbitrary detentions — during interviews with more than 120 people.

President Nicolas Maduro's government has long denied allegations of torture, accusing HRW of being part of an America-funded conspiracy to stop socialism.

"The strategy used against my country from certain centers of power is a clear example of the use of human rights as a political weapon," Foreign Minister Jorge Arreaza told the United Nations' human rights council in September.

Tensions in Venezuela escalated in March this year when the pro-government Supreme Court stripped the opposition-controlled congress of its last remaining powers, sparking near-daily protests to oust Maduro's leftist government.

Demonstrators said National Guard soldiers clamped down on protests with excessive force. Maduro said his administration faced an "armed insurgency."

Venezuela is in the grip of a severe political and economic crisis that has prompted fears of a refugee crisis in neighboring countries.

The opposition blames Maduro's for the soaring inflation and basic food shortages that have brought Venezuela's economy to ruin. But despite the public unrest Maduro refuses to step down.

Earlier this month the European Union followed the United States in implementing a number of sanctions against Venezuela, including a ban on arms sales. Foreign ministers from EU member states said the measures could be reversed if Maduro reacts to demands for more democracy in Venezuela, including the release of political prisoners.

an/sms (AP, Reuters)