Madrid, Spain - Europe's first underground illegal cigarette factory to be unearthed has been revealed, after Spanish police rescued six men who were suffocating there.

Despite shouting for help and banging on the roof of the clandestine bunker, the men - abandoned as supplies of air ran out - could not at first be heard by police searching above, because the factory was tightly soundproofed.

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Only the persistence of the officers, who used a forklift truck to remove a cargo container that hid the entrance to the factory, saved the men inside.

The factory, built 13 feet (four metres) under a stable in Monda, in Malaga Province in southern Spain, was equipped with bunk beds and living quarters for staff.

At least 3,500 cigarettes rolled off the production line every hour, police said as they released details of the operation on Thursday.

This machinery in the illegal underground tobacco factory produced 3,500 cigarettes an hour [Jon Nazca/Reuters]

The factory, which had been operating for about a year, generated profits of more than 600,000 euros ($647,000) per week.

Workers were blindfolded as they were escorted into the factory, where they would spend two weeks at a time, the Spanish Civil Guard said. They were reportedly each paid 7,000 euros ($7,550) per month.

The men were not allowed to leave the factory unaccompanied, and had to endure exposure to toxic materials in conditions that police described as "dangerous".

Spanish police, acting on a tip-off from Europol, had launched an operation, named Hannibal, to find a drugs gang based on the Costa del Sol, in southern Spain.

After a series of raids last week, they seized more than three million cigarettes, along with more than 17 tonnes of rolling tobacco, 20kg (44lbs) of hashish and 144kg (317lbs) of cannabis.

Workers were kept in 'semi-slavery', locked into the underground factory - replete with kitchenette and bunk beds - for two weeks at a time, a source told Al Jazeera [Jon Nazca/Reuters]

However, after their arrest, none of the 12 British suspects mentioned the existence of the factory to police, leaving six Ukranian workers trapped inside.

Unable to contact the outside world, they had difficulty breathing after a generator that supplied fresh air was turned off.

"If the officials had not found the clandestine factory in time, the lack of oxygen would have soon made the underground conditions incompatible with the survival of the workers who were there," a Civil Guard spokesman said.

Carlos Gallego, of the Civil Guard's Central Operative Unit, told Al Jazeera: "The gang chose the perfect place to set up their factory; it is isolated and was very difficult to find.

"There have been other illegal cigarette factories, but this is the first one in Europe that has been discovered underground."

The United Kingdom's National Crime Agency tracked down Daniel Dobbs, living in Spain under a false name, last year. In January 2014, he had been sentenced to nearly 14 years in prison, but absconded from jail [NCA handout]

The 12 British nationals arrested in the operation have appeared in court to face charges of falsification, fraud, money laundering and offences against public health connected with drugs. All were remanded in custody to face trial later.

One of the alleged ringleaders was a Briton named by Spanish media as Daniel Dobbs, 31, from Malton, North Yorkshire, a convicted heroin trafficker who has been on the run from British authorities since November 2018 - when he was found missing from his cell in an open prison.

"Dobbs's arrest is a fantastic result and shows the power of our international partnerships," the United Kingdom National Crime Agency's Alison Abbott said in a statement emailed to Al Jazeera.

"British fugitives should know that we will catch up with them, no matter where they are."

The gang was reportedly smuggling the illicit cigarettes to Britain.

The suspected leaders of the gang lived in large villas on the Costa del Sol, while the men who worked in the factory lived in conditions of "semi-slavery", a source at the financial crimes unit at Spain's Civil Guard told Al Jazeera.

"They were paid, but lived in unhealthy conditions and were locked into this place for 24 hours a day for two weeks at a time," said the source, who asked not to be named.