Summary:Mexican troops have captured one of the country's most wanted drug-gang leaders.
Mario Ramirez Trevino, known as X-20, is said to be the head of the cocaine and marijuana-smuggling Gulf Cartel.
Security was stepped in the north of the country to prevent rival gangs fighting to expand their influence into the void left by the capture.
It is the second high-profile arrest since President Enrique Pena Nieto came to power last December.
Correspondents say that has led to questions about how different Mr Pena Nieto's strategy is to his predecessor Felipe Calderon's.Big-name targets
Ex-President Calderon deployed the army across the country and pursued cartel leaders. Many blamed the tactics on the surge of violence seen in the country since.
However, Mr Pena Nieto promised to change the policy by tackling cartels through law enforcement on a local level rather than the capture of big-name targets.
The US government was offering a reward of $5m (£3.2m) for information leading to the capture of Mr Ramirez, while Mexico offered about $3m.
Ramirez Trevino is thought to have taken over as leader of the Gulf Cartel after the arrest of Jorge Eduardo Costilla, known as The Coss, last September.
Mexican media reported that the drug lord was arrested by a joint Army and Marines operation in Rio Bravo, in the northern Mexican state of Tamaulipas.
The Interior ministry confirmed the arrest on Twitter.
"This morning, the Mexican Army has captured Mario Armando Ramirez Trevino," it said.
On Sunday, a government official told there was now a power vacuum in several of the cartels, which could leader to infighting.
"The government has reinforced the security strategy with the aim of neutralising any eventual actions by other criminal organisations in this region," interior ministry spokesman Eduardo Sanchez said.
The leader of the Gulf Cartel had been wanted by US authorities since 2006.
He is considered "at least as violent" as the leader of the notoriously brutal Zetas cartel, Miguel Angel Trevino Morales, who was also arrested last month.
Analysts say the authorities believed Ramirez Trevino had enough influence to attempt unifying the Gulf and Zeta cartels, which used to be one organisation until its leadership split up in 2010.
The Zetas began as the enforcement arm of the Gulf Cartel.