By Stephanie Nebehay

GENEVA (Reuters) - Venezuela's Supreme Court has progressively dismantled the rule of law, becoming an instrument of President Nicolas Maduro's government in what amounts to a coup against the constitutional order, an international human rights group said on Tuesday.

The collapse of the judiciary has left victims of torture, killings and disappearances and their families without recourse to justice after months of violent street protests, the International Commission of Jurists (ICJ) said. It called on the U.N. Human Rights Council to take action.

"We have seen a judiciary that has essentially lost its independence and become a tool of a very authoritarian executive branch," Sam Zarifi, ICJ Secretary-General, told a news briefing.

"This breakdown of the rule of law has also severely obstructed accountability (and) essentially made it impossible to bring to justice those responsible for gross violations of human rights," he said.

The Venezuelan government did not immediately reply to a request for comment. Foreign minister Jorge Arreaza on Monday rejected as "baseless" a U.N. report that found excessive use of force by its security forces and other violations.

Four months of demonstrations in which at least 125 people were killed have all but stopped due to fatigue among protesters and disillusionment at seeing the ruling Socialist Party cement vast powers despite the concerted opposition push.

The ICJ said the top court had undermined human rights and infringed the Constitution through a series of rulings since December 2015.

In two rulings in March 2017, the Supreme Court of Justice "effectively claimed legislative powers for itself, depriving the National Assembly of its Constitutional powers and granting sweeping arbitrary powers to the executive," it said.

"These decisions amount to a coup d’état against the Constitutional order and have ushered in a new reign of arbitrary rule," Zarifi said.

Judges on the Supreme Court are mainly from the Socialist Party or former officials of the government of Maduro or both, the Geneva-based jurists' group said.

"Judges who have demonstrated independence and ruled against the executive branch have faced retaliation and punishment," Zarifi said.

MADURO DENIES POWER GRAB

Maduro denies accusations of a power grab, saying his actions, which include the creation of an alternative Constituent Assembly that has granted itself law-making powers, aim to restore peace after months of protests and violence.

The new Constituent Assembly "at this point acts as a body outside of the rule of law. It is able to legislate and create law and new regulations in the country without accountability," Zarifi said.

The ICJ report, "The Supreme Court of Justice: an instrument of executive power", was issued on the sidelines of the U.N. Human Rights Council.

U.N. human rights chief Zeid Ra'ad al-Hussein said on Monday that Venezuelan security forces may have committed crimes against humanity against protesters and called for an international investigation.

"The evidence that's there, of course it is not adjudicated, but certainly suggests room for investigating crimes against humanity," Zarifi said.

Such crimes are defined as grave and systematic violations including torture, enforced disappearances, and extrajudicial killings that are part of a state's policy, he said, adding: "Those indicators are all there in Venezuela."

Carlos Ayala, a Venezuelan lawyer and ICJ commissioner based in Caracas, told the briefing: "The situation is worsening on a daily basis because of hyper-inflation (and) the lack of access for the majority of the population to medicines and health care.

"More than 600 civilian students are currently being tried before military courts in Venezuela, more than 1,000 young persons, students, are in jail because of the demonstrations."

(Editing by Matthew Mpoke Bigg)