Baghdad (CNN) -- A notorious prison in Baghdad's Green Zone is still open despite the Iraqi government having said more than a year ago it was closed, Human Rights Watch said on Tuesday.
The report about the Camp Honor prison facility, which is in the highly fortified area of the city's center, also detailed evidence of mass arrests. The Iraqi government denied the allegations.
"Iraqi security forces are grabbing people outside of the law, without trial or known charges, and hiding them away in incommunicado sites," said Joe Stork, Human Rights Watch's deputy Middle East director.
"The Iraqi government should immediately reveal the names and locations of all the detainees, promptly free those not charged with crimes, and bring those facing charges before an independent judicial authority," he said.
The report comes just months after the United States withdrew troops from the Middle Eastern country at the end of 2011.
Human Rights Watch identified two main waves of detentions -- in October and November and the again in March ahead of the Arab League summit in Baghdad.
The New York-based rights group reported last year that detainees at Camp Honor were held in inhumane conditions and tortured. Legislators visited the site in response to that report, and in March 2011 the government announced it had closed the prison, Human Rights Watch said.
According to Iraqi government officials, it remains closed.
"There is no secret prison in Iraq ... All Iraq's prison and detention centers across the country are subject to inspections at any time by the Ministry of Human Rights," said Ali al-Moussawi, an adviser to Prime Minister Nuri al-Maliki.
Human Rights Ministry spokesman Kamel Ameen said an inspection team from the ministry visited what was Camp Honor less than two weeks ago and found nothing but an abandoned building.
He also denied the reports of mass arrests, stressing that security forces cannot make arrests without the proper warrants.
Human Rights Watch said it interviewed more than 35 former detainees, family members, lawyers, legislators and government officials to compile its report. All asked to remain anonymous.
"It's a matter of grave concern that Iraqis in so many walks of life, officials included, are afraid for their own well-being and fear great harm if they discuss allegations of serious human rights abuses," said Stork.
CNN's Mohammed Tawfeeq contributed to this report.