At least 80 political prisoners were freed on Sunday in Venezuela, where detainees are slowly being released under pressure from the United States, the non-governmental organisation Foro Penal said.

The group's director Alfredo Romero said on X that the NGO was verifying the identities of those released at jails nationwide, adding that it was "likely that more releases will take place."

Foro Penal attorney Gonzalo Himiob said the releases had taken place in the early hours.

"This figure is not yet definitive and could increase as we proceed with verifications," he said, also on X.

The government of Venezuela's interim leader Delcy Rodriguez, who took power when Nicolas Maduro was captured by US special forces early this month, has promised to release a "large number" of the hundreds of Maduro opponents languishing in prison.

Authorities in Caracas say they have freed 626 detainees since December, but Foro Penal has only recorded about half as many releases in that time.

Venezuela's political opposition and human rights groups have criticised the slow unfolding of the process.

Loved ones of detainees have been camped out outside prisons waiting for their release. (AFP)