More than 250 packages of drugs were found in the submarine
The Mexican navy says it has seized nearly six tonnes of cocaine found inside a 10m-long (31ft) makeshift submarine in the Pacific Ocean.
A naval spokesman said they had known about such submarines but this was the first time they had seen one. US intelligence helped in the operation.
Four Colombian crew members have been taken into custody.
In a separate development Mexico's army seized at least 12 tonnes of marijuana in the city of Tijuana.
The haul is the largest since President Felipe Calderon sent thousands of troops to the region along the border with the US.
The submarine had been carrying its cargo from Colombia towards the coast of Mexico when it was intercepted on Wednesday. It took the navy two days to tow it to shore.
The cocaine was inside sealed packages on board the sub
The US Homeland Security Secretary, Michael Chertoff, revealed on Friday that US intelligence had led Mexican forces to the submarine.
"We shared information with the Mexican navy, but the Mexican navy acted alone. It was actually their seizure, their marines, their helicopters and naval vessels that captured the submarine," Mr Chertoff said on a visit to Mexico City.
Mexican Vice-Admiral Jose Maria Ortegon Cisneros said that the drugs were in 257 packages wrapped in black plastic.
He said that the submarine was equipped with a global positioning system (GPS) and a compass.
"This is going to force us to intensify our aerial surveillance, because the freeboard (distance from the deck to the water) of this sub is not detected by radars or any type of electronic detection device," he told reporters in Salina Cruz in the state of Oaxaca.
"Four people were detained. Based on their statements they say they left from Colombia. They did not specify from what port. Three of them are from Buenaventura and the other from a town in the centre of Colombia."
He added that the navy had stepped up patrols in the area where the sub was seized.
The crew claimed to be fishermen forced by drug cartels to move the cargo, said Vice Admiral Cisneros. They said they had sailed from Colombia about a week ago.
Meanwhile, the 12 tonnes of marijuana seized in the border city of Tijuana was the single biggest haul of the drug since President Felipe Calderon sent more than 25,000 troops to the region last year to tackle trafficking.
Officials said that about three tonnes of the drug were found in a stolen car on Thursday while the rest was discovered on Friday.
So far in 2008 the Mexican army says it has confiscated more than 500 tonnes of marijuana in the state of Baja California alone.
Mr Calderon came to power in 2006 promising to rid Mexico of organised and drug-related crime.
But the BBC's Americas editor, Emilio San Pedro, says that despite celebrations at the military barracks in Tijuana, this seizure - no matter how large - is mostly a symbolic victory as it is far from a death blow to the cartel bosses.
Authorities say the traffickers are extremely organised and far from willing to give up their lucrative enterprise, which earns them tens of millions of dollars every year.
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