El Salvador has moved approximately 2,000 prisoners, most of them suspected gang members, to a newly opened 40,000-person mega-prison.
Photos of prisoners displayed on the Internet showed the highly tattooed, barefoot men with shaved heads stripped down to white boxer shorts.
President Nayib Bukele has declared a “war” on violent gangs in the Central American country, and the mega-prison is part of that campaign.
He posted on Twitter: "This will be their new home, where they won't be able to do any more harm to the population.”
Human rights groups have objected to the campaign because they say innocent people have been caught up in it, with some dying before they could prove their innocence.
The campaign, however, remains popular with many Salvadoreans, reports say.