GENEVA (Reuters) - Venezuelan security forces have systematically wielded excessive force to suppress protests, killing dozens, and have arbitrarily detained 5,000 people since April, including 1,000 still in custody, the United Nations human rights office said on Tuesday.

Members of security forces stand guard during clashes with demonstrators near Fuerte Paramacay military base in Valencia, Venezuela August 6, 2017. REUTERS/Andres Martinez Casares

It called on the government of President Nicolas Maduro to rein in security forces and investigate alleged abuses and release people from arbitrary detention.

On Friday, Venezuela inaugurated a new legislative superbody that is expected to rewrite the constitution and give vast powers to Maduro’s ruling Socialist Party, defying protests and worldwide condemnation that it undermines democratic freedoms.

“We are concerned that the situation in Venezuela is escalating and these patterns of human rights violations are showing no signs of abating,” U.N. human rights spokeswoman Ravina Shamdasani told a news briefing in Geneva.

The ousting of Attorney-General Luisa Ortega, an outspoken government critic, was a sign of an “increasing undermining of democratic institutions and of independent institutions”, she said, calling on the government to ensure her safety.

On Tuesday, Venezuela’s Supreme Court ordered the arrest of opposition mayor Ramon Muchacho of the Caracas district of Chacao, the site of intense anti-government protests.

U.N. High Commissioner for Human Rights Zeid Ra’ad al-Hussein said in a statement: “These violations have occurred amid the breakdown of the rule of law in Venezuela, with constant attacks by the Government against the National Assembly and the Attorney-General’s Office.

“The responsibility for the human rights violations we are recording lies at the highest levels of Government.”

U.N. human rights officers were not allowed into Venezuela, but issued preliminary findings based on 135 interviews in Panama and from Geneva in June and July with victims, families, witnesses, doctors and lawyers. They also received information from the Venezuelan ombudsman’s office.

Of 124 deaths investigated, they found at least 46 attributable to security forces and 27 to pro-government armed groups, with the rest unclear.

“Witnesses spoke of security forces firing tear gas and buckshot at anti-Government protestors without warning. Several of the individuals interviewed said tear gas canisters were used at short range, and marbles, buckshot and nuts and bolts were used as ammunition,” Shamdasani said.

Ill-treatment and torture have been reported in detention, including beatings, use of electric shocks, hanging detainees by their wrists and “suffocation with gas”, she said.

“Journalists have reportedly been shot at with tear gas canisters and buck shot and been detained, threatened and had their equipment taken on several occasions,” she added.