(Adds Holt comments)
By Luc Cohen and Alexandra Ulmer
CARACAS, May 16 (Reuters) - Inmates at a crowded Caracas detention center revolted on Wednesday, with jailed opponents of Venezuelan President Nicolas Maduro and a Mormon missionary from Utah begging for freedom and medical attention in postings on social media.
There was no official information on the incident, but in videos posted on social media men identifying themselves as prisoners said they had taken over the headquarters of intelligence agency Sebin, known as the Helicoide, where hundreds of people are held.
"This has been taken over peacefully by all the political prisoners and all the prisoners who are abducted here, who are tortured daily," a man said in one of the videos. He said tear gas and weapons had been fired at detainees but they were holding out to demand freedom.
Reuters was unable to independently confirm the origin of the videos or circumstances under which they were made.
Venezuela's Information Ministry did not respond to a request for comment.
Chief Prosecutor Tarek Saab tweeted, "In the face of the events that happened today in the Sebin headquarters at the Helicoide, we sent a commission of the prosecutor's office to the facility. That delegation spoke to a representative of the prisoners to respond to their requests."
In a midafternoon Facebook post, Joshua Holt, a U.S. citizen and missionary whose family has said he was framed on weapons charges while in Venezuela for his wedding, said, "Helicoide the prison where I am at has fallen the guards are here and people are trying to break in my room and kill me. WHAT DO WE DO?"
In a video seen on Twitter late on Wednesday Holt said, "I'm here to show you that I am not being kidnapped. The only people who are kidnapping me is the government of Venezuela. We need the people to help us." He was flanked by three other men.
He said all four of them were being detained without trial and that some detainees were being denied medical attention.
His mother Laurie Holt told Reuters that she did not know the sequence of the videos and was unable to confirm Holt's current situation.
Activists said the incident had been precipitated by the beating of activist Gregory Sanabria from the state of Tachira. He appeared with a bruised face in pictures on social media.
Rights groups and Maduro opponents have said several hundred political prisoners have been unfairly jailed. Maduro has said all jailed activists were being held on legitimate charges of violence and subversion.
The U.S. embassy in Caracas said it was "very worried" about the situation at the Helicoide.
"Joshua Holt and other U.S. citizens are in danger. The Venezuelan government is directly responsible for their security and we will hold them responsible if anything happens to them," the embassy tweeted in Spanish.
Todd Robinson, the chargé d'affaires at the U.S. embassy, went to the Venezuelan Foreign Ministry for information, the embassy added. "No response from the government." (Additional reporting by Leon Wietfeld and Vivian Sequera Writing by Alexandra Ulmer Editing by Toni Reinhold)
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