The UN rights chief warned Monday of possible "crimes against humanity" in Venezuela, prompting the crisis-wracked country to accuse his office of wielding human rights as "a political weapon".
Venezuela's crisis has caused food and medicine shortages, deadly unrest and calls for President Nicolas Maduro to quit.
Clashes with security forces at anti-government protests have left around 130 people dead since April.
"My investigation suggests the possibility that crimes against humanity may have been committed," Zeid Ra'ad Al Hussein said at the opening of the 36th session of the Human Rights Council, the United Nations' top rights body.
International powers accuse Maduro of dismantling democracy by taking over state institutions in order to resist the opposition pressure.
"There is a very real danger that tensions will further escalate, with the government crushing democratic institutions and critical voices," Zeid warned, calling for an international probe.
He said an investigation by his office had noted the widespread use of "criminal proceedings against opposition leaders, recourse to arbitrary detentions, excessive use of force and ill-treatment of detainees, which in some cases amounts to torture."
- 'Bias' -
Venezuelan Foreign Minister Jorge Arreaza Montserrat also addressed the council Monday, demanding that Zeid stop his office's "aggressions" towards Venezuela, and slamming "the political, selective and bias use of human rights."
"The strategy used against my country from certain centres of power is a clear example of the use of human rights as a political weapon," he told the council.
He warned that recent reports from the UN rights office "have no methodological rigour, they are baseless and they are trying to upset our sovereignty, peace and stability."
Late last month, Zeid echoed international concerns that Venezuela was slipping into dictatorship, cautioning that democracy in the country was "barely alive, if still alive."
In a report, his office at the time accused Venezuelan authorities of implementing a policy of systematic repression and excessive force with the aim to "crush dissent and instil fear in the population" to curb the protests against Maduro.
The tactics listed in the report included "the use of electric shocks, severe beatings, stress positions, suffocation, and threats of sexual violence and death."
The report also criticised Venezuela's all-powerful constituent assembly and its "truth commission", which has been tasked with investigating several opposition leaders for treason.
Julieta Lopez, the aunt of one of Venezuela's top jailed opposition figures Leonardo Lopez, warned Monday that the country "cannot survive without international assistance."
- Dictatorship 'too soft' -
"Dictatorship is too soft a word," she told reporters on the sidelines of the rights council, maintaining that "Venezuela is being run by a gang of organised criminals."
Zeid stressed the need to hold perpetrators of rights abuses in Venezuela accountable, telling the council Monday he supported the concept of a truth commission.
But "the current mechanism is inadequate," he said, urging that "it be reconfigured with the support and involvement of the international community."
He called on the rights council "to establish an international investigation into the human rights violations in Venezuela."
Zeid also pointed out that Venezuela currently holds one of the 47 rotating seats on the Human Rights Council, and thus has a particular duty to "uphold the highest standards in the promotion and protection of human rights."
Without naming Venezuela specifically, he called on the council to consider "the need to exclude from this body states involved in the most egregious violations of human rights."
Zeid received support from 116 non-governmental organisations, mainly from Latin America, who on Monday issued a joint statement insisting Venezuela should be among the rights council's top priorities.
"Member states should send a clear message to the Venezuelan government that these abuses are not tolerated and those responsible of abuses will eventually be brought to justice," the statement read.
The International Commission of Jurists meanwhile cautioned that Venezuela's Supreme Court had "ceased to act as an independent court upholding the rule of law, but has become an arm of an authoritarian executive."