Soldiers and heavily-armed police took control of Mexico's highest-security jail on Friday amid fears drug gangs were planning a mass breakout.
More than 750 troops were used at La Palma jail, just west of Mexico City.
Two recent fatal shootings at the prison were blamed on rival jailed drug lords, who are now thought to have formed an alliance and plan to escape.
President Vicente Fox has made cracking down on organised crime and corruption a priority.
Troops and police surrounded the prison at about on Friday morning as part of an operation to separate alleged drug lords Osiel Cardenas and Benjamin Arellano Felix.
The men, reputed to run two of Mexico's largest drug cartels, were understood to have joined forces inside the prison in order to fight another drug organisation.
Breeding ground
Police had become worried over security when a weapon was smuggled in and used to assassinate an associate of Joaquin Elchapo Guzman, a notorious escaped drugs baron.
Two weeks later Joaquin Guzman's brother, Arturo Guzman, was also shot and killed.
"In the past three years, the maximum security has begun to wane, We have to restore it," an official from Mexico's public safety department said.
Many of the inmates at La Palma belong to one of 11 drug gangs that operate inside the prison, Mexican authorities said.
The BBC's Claire Marshall in Mexico City says the country's prisons are known as a breeding ground for corruption.
This high-profile action comes after the former head of Mexico's prisons resigned on Thursday after coming under increasing pressure to improve the penal system, she adds.
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