Some 54 opposition prisoners are taking part in the protest since Wednesday at the Sebin intelligence service's Helicoide center in Caracas, prisoners rights group Foro Penal said.
An American man among them, Joshua Holt, pleaded for help from the US government in a video message via Twitter early Thursday.
"I'm calling on the people of America. I need your help to get me out of this place," said Holt, who was detained in 2016.
"I've been begging my government for two years. They say they've been doing things but I'm still here."
Holt was arrested by Sebin and accused of possessing weapons and plotting to destabilize the government of President Nicolas Maduro. Top officials in the socialist government have labelled him a US spy.
US embassy official Todd Robinson went to the foreign ministry late Wednesday to request information about Holt but said he had not received a response.
On his Twitter account, Daniel Ceballos, a former opposition mayor of the city of San Cristobal, published a photo of a group of prisoners, including Holt, gathered inside the detention center.
The prisoners are demanding the immediate release of 20 inmates who have been ordered free on bail, speedier court processes and medical care.
State prosecutor Tarek William Saab said the situation inside the prison was calm and that the men were in their cells.
Saab told CNN's Spanish-language service that negotiations were underway between his office, the ministry for penitentiary affairs and Sebin "to evaluate the cases that may have experienced a procedural delay".
Relatives have gathered outside the jail waiting for news.
"The protest is peaceful, they do not have weapons," said Jorge Fernandez, father of one of the prisoners detained since 2016. "What we demand is that justice be done."
Ceballos' wife Patricia accused Saab of lying about the situation being calm inside the center.
"The situation is the same, the only information that we have is unofficial, from audios that come from inside. We know that the internal protest is being maintained but we have no information about the detained," said Alfredo Romero, head of Foro Penal.
The incidents come ahead of Sunday's presidential election in which socialist incumbent Maduro, who has been accused of using the Sebin to crack down on opponents, is seeking a second six-year term.
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