Human Rights Watch said Thursday it was concerned that Yemen's Huthi rebels were carrying out "abusive practices" against former allies -- loyalists of slain ex-president Ali Abdullah Saleh -- and civilians.
The Huthis gunned down Saleh on Monday, and the Iran-backed insurgents have since consolidated their grip on the capital.
"The Huthis should remember that the killing of Ali Abdullah Saleh does not negate their obligations under international law, nor to civilians in areas under their control," HRW's Yemen researcher Kristine Beckerle told AFP.
Beckerle said the actions of the Huthis fit a pattern -- documented by HRW since the group took over Sanaa in 2014 -- of arbitrary detentions and forcible disappearances against critical voices, notably activists and journalists.
"Troubling reports from Sanaa over the last few days indicate the Huthis are again engaging in these abusive practices," she said.
A three-year alliance between the Huthis and pro-Saleh forces has degenerated into open warfare in Sanaa since last week, leaving at least 234 people dead and 400 wounded, according to the Red Cross.
The Huthis seized near total control of the capital after Saleh's death but even as the violence subsided, rivals of the insurgents have reported a massive crackdown.
Members of Saleh's political party said the Huthis have rounded up hundreds of suspected loyalists, including his family members.
Reports of summary executions could not immediately be confirmed.
Tribal chiefs in at least four provinces have been summoned to pledge allegiance to the Huthis, and the crackdown has also targeted local journalists.
On Wednesday, the Committee to Protect Journalists as well as Reporters Without Borders urged the Huthis to immediately release 41 staff of Yemen Today, a pro-Saleh television station.
Yemen's Sanaa-based national syndicate of journalists said the Huthis had posted the names of employees of media viewed as hostile at checkpoints around the capital.