HRW wrote that it had recorded "some information" about seven abducted people and two arrested persons.
"But in nine other cases, families, friends, and lawyers of people kidnapped or detained at or after they participated in protests in Baghdad, Karbala, and Nasriya, said they were too frightened or worried about the consequences for the detained person to provide details."
At least seven people, including a 16-year-old boy, have gone missing from or near Tahrir Square since October 7, while taking part in ongoing demonstrations in the Iraqi capital, HRW said in a report on its website.
Four people are still missing today, their families told the rights watchdog.
They had visited police stations and office of relevant authorities to seek information to no avail.
HRW said the government has not taken concrete action to locate the missing.
Regarding those responsible for the abductions, the organization noted that it was not clear whether government security forces or "armed groups" had been behind them.
"In another two cases, security forces arrested and arbitrarily held protest supporters," HRW added.
"Whether the government or armed groups are behind the abductions in Baghdad, the government bears the responsibility for keeping people safe from such targeting," said Sarah Leah Whitson, Middle East director at HRW.
"Authorities are failing Iraqi citizens by allowing armed forces to abduct people, and it will be up to the government to take swift action against these abuses."
"Iraqi authorities should ensure an independent investigation into all abductions," the report stated.
"The authorities should release all protesters who have not been charged with a recognizable criminal offense or anyone detained solely for exercising their right to peaceful assembly and protest."
HRW added further that the alleged perpetrators unlawful detention "should be investigated and prosecuted, including both state security forces and private individuals."
Since they began in early October, widespread protests have resulted in the deaths of nearly 400, while some 16,000 have been injured, according to Iraq’s High Commission for Human Rights.
The UN, as well as Amnesty International, have called for an end to the bloodshed and urged security forces to show restraint against protestors.
Demonstrators in Iraq are calling for radical change in Iraq’s political system, which they say fails to address their needs but, rather, serves the interests of a small governing elite.
The developments have forced Iraqi Prime Minister Adil Abdul Mahdi to hand in his resignation.
In a special session held on Sunday in Baghdad, the Iraqi Parliament voted to accept Abdul Mahdi’s resignation.
Editing by Nadia Riva