Alejandro Saenz takes a seat behind the shabby prison desk with the confident air of a CEO. Tall and thin with gelled hair, he looks like a conceited teenager. He is actually 30, a killer, and has been imprisoned in the Ciudad Juárez jail for ten years.

“We are very good people,” said his colleague Nicolas Sosa, and it is hard to tell if he is joking. Slick and muscled, with patterns shaved into his beard and wearing a tight white t-shirt and cowboy boots, Sosa’s looks are marred only by his nose, which is bent to one side. “I am here because I killed two cops,” he said. “I was mad — they stopped me and took my money, and I had a gun.”

Sosa and Saenz are two senior members of the Artist Assassins, a drug gang working in Ciudad Juárez, the most violent city in the world. The 600 Artist Assassins and 1,200 Mexicles, another local gang, are employed by the Sinaloa Cartel — run by Joaquín “El Chapo” Guzmán, the country’s most wanted man — to control the drug traffic passing through Juárez.

But the Artist Assassins and the Mexicles are in the middle of a war with the Aztecas. With 7,000 members the Aztecas are the most powerful drug gang in Juárez and work for the rival Juárez Cartel. The leaders of these cartels are businessmen in hiding and the gangs act as their enforcers on the streets. “The Aztecas have a very different way of thinking to us,” Sosa said. “We live and work very closely. We have values: respect, and the liberty to choose. They don’t. They kill our families, our friends, our kids.

“We’re not a gang — we’re a union. The difference is you can get out if you want to. You don’t have to stay.”

Sosa is soliciting a little too much sympathy. “Those guys are mean criminals,” said the Mayor of Juárez, José Reyes Ferriz, who accompanied The Times on a visit to the prison this week. “The Artistas Asesinos feel that killing is an art form.”

Mr Reyes Ferriz has been in office since 2007. His popularity is mixed and not helped by rumours that he lives safely across the border in El Paso — a rumour he denies. Two weeks ago a plastic bag was found near his office with a pig’s head inside and a sign next to it declaring that the mayor would be dead within two weeks. But threats are commonplace for Mr Reyes Ferriz, from the drug gangs and former police officers that he has fired.

He is proud of having cleaned up the corrupt local police force. “I just fired 23 more officers,” he said, back in his grand, wood-panelled office built by his father, who was also a mayor of Juárez. Its windows overlook the corner where the US consulate worker Lesley Enriquez and her husband were killed by Aztecas two weeks ago.

“Since I took office I’ve fired 1,000 people who have failed our confidence test. Four out of five police officers in Juárez were hired by me,” he said.

The mayor has presided over the city at a time when two major turf wars claimed thousands of lives. The first, the result of a falling-out between the Juárez and the Sinaloa cartels, began in 2008. “At the end of December we started getting information that there was going to be a big fight coming on January 6 — the last day of the Christmas holiday season in Mexico — and there were going to be a lot of people dead,” the mayor said.

“Actually the killings started the day before and that fight continued for the whole of 2008. We had 1,600 people die in that war.”

When in February last year the murder rate climbed to ten victims a day, President Calderón decided to send in more than 5,000 troops. The rate of murder slowed but after three months of calm the killings resumed.

“We didn’t understand it then but what had happened was that the drug cartels had decided not to ship cocaine through Juárez any more,” the mayor said. “This had become one of the most patrolled cities in the country and because the flow of cocaine comes from Colombia it doesn’t have to come through Juárez.

“There are many routes it could take. They’re not going to send an expensive shipment of drugs through a place that’s patrolled by 11,000 people.”

When the gangs were deprived of their main source of income a second war erupted over the remaining trade. “You lack money. You’re hungry, you have to eat and look after your wife, your kids,” Azteca Rigoberto, 30, said. He has served 18 months for robbery and hardly comes across as a family man.

Each gang has its own style, from their clothes and tattoos to their expressions. While the Artist Assassins are groomed and eloquent, the Aztecas are pallid, wear oversized jeans and padded jackets and seem slightly crazed. The Mexicles are perhaps the most normal-looking and are reserved and polite.

The gangs have to be kept apart in prison and when they arrive inmates are asked with which group they would like to be housed. In March last year the Aztecas escaped from their cells and started a fight, in which 20 Mexicles and Artist Assassins were killed.

Ciudad Juárez owes its murder rate — an average of six dead a day — to this rivalry. The gangs have decapitated bodies and hung them from bridges, killed children, pregnant women and, in February, 15 teenagers. They regularly kidnap for ransom and they extort nearly every business in Juárez. When one funeral home refused to pay it was burnt down and its owners shot.

Only 200 of the murders in Juárez last year were civilians. Three quarters of the dead were gang members between the ages of 14 and 24, and were identified by their tattoos.

Tattoos have become the mark of the gangster in Juárez — the 400 foreign-owned factories in the city refuse to employ anyone with one. “You have tattoos on your body, you won’t get a job,” Saenz said.

Mayor Reyes Ferriz is trying to solve this problem by finding a company that will take on ex-convicts. “I’ll buy the plant, I’ll pay the manager,” he said. But it is difficult to persuade these gangs, who know how lucrative drugs can be, to take up an honest profession. With $1 million (£670,000) of drug money still circulating daily in Juárez — a city where the starting salary at one of the many factories is $40 a week — joining a drug gang is the way to make money fast.

“It’s a social problem,” said Jesús, a 30-year old Azteca who wears a rosary and one black glove. He is in jail for assaulting a shopkeeper. “If your parents are at the factory working all day, you look for a life on the streets. You just can’t meet violence with violence — that’s the Government’s big error.”