(Adds details on Holt, prisoner comments, U.S. reaction)
By Luc Cohen and Alexandra Ulmer
CARACAS, May 16 (Reuters) - A riot erupted on Wednesday at a crowded Caracas detention center and jailed opponents of Venezuelan President Nicolas Maduro and a Mormon missionary from Utah begged for freedom and help in postings on social media, according to videos and relatives.
There was no official information on the incident, but videos from prisoners and comments by family members and activists suggested it had been precipitated by the beating of a young activist, Gregory Sanabria, from the state of Tachira.
Alfredo Romero, a lawyer with the Penal Forum rights group which provides legal representation for prisoners, met relatives of prisoners in front of the headquarters of intelligence agency Sebin, known as the Helicoide, where hundreds of people are held. Romero said the beating appeared to have sparked "some of sort of internal fight."
Several activists incarcerated at the Helicoide posted videos on social media and said the jail had been taken over to protest conditions.
"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 who identified himself as a prisoner said in a video.
He said authorities had fired weapons and tear gas but detainees were holding out to demand freedom. Photos of a man with a bruised face who was identified as Sanabria were also posted on social media.
Venezuela's Information Ministry did not respond to a request for comment, but its chief prosecutor said in a tweet that his office was on site, without providing details.
"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," chief prosecutor Tarek Saab tweeted.
Rights groups and Maduro opponents have said several hundred political prisoners were unfairly jailed, while Maduro has said all jailed activists were being held on legitimate charges of violence and subversion.
In another video on social media, Joshua Holt, a U.S. citizen and Mormon missionary whose family has said he was framed on weapons charges while in Venezuela for his wedding, pleaded to be set free.
"I'm calling on the people of America: I need your help to get me out of this place. I've been begging my government for two years ... And now my life is threatened," Holt said.
A post on his Facebook page read, "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?"
His mother, Laurie Holt, confirmed the page belonged to her son.
The U.S. embassy in Caracas said it was "very worried" about the situation at the Helicoide, which was originally designed to be a mall.
"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 tweeted. "No response from the government." (Additional reporting by Leon Wietfeld and Vivian Sequera Writing by Alexandra Ulmer Editing by Toni Reinhold)
Our Standards: The Thomson Reuters Trust Principles.