HRW said on Tuesday Saidjahon Zainabitdinov was held as part of a government effort to suppress details of the 13 May bloodshed in the city of Andijan when troops were believed to have killed hundreds of protesters.
Witnesses said about 500 people were killed when troops fired on a protest, but Uzbek President Islam Karimov said the toll was 169 and most were "bandits." He has rejected international calls for an inquiry.
"The government is trying to silence activists through this arbitrary arrest and intimidation," Holly Cartner, HRW's Europe and Central Asia director, said in a statement.
"The Uzbek government should release him immediately ... The international community must call on (Uzbekistan) to ensure the safety of Zainabitdinov and other ... activists who could be subject to repercussions," she said.
The Uzbek authorities have made no comment about the arrest or on the HRW statement.
"The government is trying to silence activists through this arbitrary arrest and intimidation"
HRW's Europe and Central Asia director
Most of the killings were outside Andijan's School Number 15, where witnesses said troops fired on a crowd of protesters who included women and children.
"There were corpses scattered behind the school fence. I counted about 400," Gulbakhor Turayeva, a local doctor, told reporters. She said some survivors were being denied proper medical care.
Uzbekistan is an ally in the US war on terror and leases its Khanabad air base to the US military, which uses it to supply its operations in neighbouring Afghanistan.
Washington, with the United Nations and the European Union, has told Karimov he faces international isolation and the prospect of having aid cut off unless he allows a full inquiry.
About 500 Uzbeks who fled Andijan, some of them with gunshot wounds, are living in a makeshift refugee camp in Kyrgyzstan, which shares a border with Uzbekistan. They say they are too scared to return home.
UN concern for refugees
The UN High Commissioner for Refugees raised concerns about the safety of Uzbek refugees who fled the recent bloody military crackdown in their country to take shelter in a camp in neighbouring Kyrgyzstan.
"We are still concerned about the safety and well-being of a group of 540 Uzbeks, including 91 women and children, who arrived in Kyrgyzstan ten days ago in the aftermath of the military crackdown," the UNHCR said in a statement published in Moscow.
Uzbek refugees say they are
too scared to return home
The Kyrgyz authorities have given the refugees assurances they will not be forced to return to Uzbekistan, the United Nations refugee agency said in Geneva on Tuesday.
Zainabitdinov was arrested in Andijan on Saturday, but his family were not informed until Monday, and the charges against him are not known, HRW said.
Long before the violence in Andijan, foreign governments and rights groups complained that Karimov's rule was authoritarian and repressive. His refusal to relinquish state control over the economy had also been criticised.
He has defended his tactics by claiming that Uzbekistan is under threat from groups aligned to Afghanistan's Taliban and want to establish Islamic rule across oil and gas-rich Central Asia.
His critics say he exaggerates the threat, but that where such groups do exist, Karimov's repressive rule is to blame for radicalising young men and driving them into the arms of such groups.
Meanwhile, authorities in Uzbekistan's Andijan province are still holding 52 people allegedly linked to violence in the eastern city that is feared to have left hundreds dead earlier this month, a top prosecution official said.
The 52 are among 97 people arrested by Andijan's law enforcement agencies on suspicion of "organising terror acts and bandit attacks" after an explosion of violence there on 13 May, Andijan's chief prosecutor, Bakhodir Dekhanov, said on national television.
Witnesses say Uzbek soldiers
fired at unarmed civilians
The other 45 have been released on condition that they report regularly to police, Dekhanov said.
President Islam Karimov has blamed the violence on militants based in neighbouring Afghanistan and Kyrgyzstan, and has vowed to pursue those responsible.
Witnesses: Indiscriminate fire
He has said that 169 people died in the events that began when armed men stormed a police station, a prison and a military base, and that no civilians died at the authorities' hands.
Independent witnesses say state forces fired indiscriminately on unarmed civilians who gathered in Andijan's central square, apparently supporting the uprising.
Such witnesses have put the true toll as high as 1000.