Hundreds of heavily armed soldiers fanned out across Mexico's bloodiest drug war city on Tuesday, trying to prevent a collapse in law and order just south of the U.S. border.

Sirens blared as the army staged one of its biggest troop build-ups in years in Ciudad Juarez, a desert city across the border from El Paso, Texas, where near-daily clashes between drug gangs and police have terrified residents.

Infamous in the 1990s for the unsolved murders of hundreds of women, Ciudad Juarez is now engulfed in the worst drug violence in Mexico as cartels in league with corrupt cops fight over one of the country's most profitable smuggling routes.

More than 2,000 people have been murdered in the area over the past year and drug gang hitmen showed their power last month by forcing the city's police chief to resign with a threat to keep killing police officers until he quit.

"We've got to show we can achieve security in Juarez, for Mexico's sake, for its economy, for people's lives, for our international reputation," said Victor Valencia, the Chihuahua state governor's representative in Ciudad Juarez.

Ciudad Juarez is prized for its location smack in the middle of the 2,000 mile (3,200 km) border with road and rail links deep into the United States. The Pacific-coast Sinaloa gang, led by top fugitive Joaquin "Shorty" Guzman, is one of several fighting for control of the city.

Mexico's police forces are tainted up to the highest levels by corruption and direct links to the drug cartels, and President Felipe Calderon has staked his reputation on a nationwide army-led crackdown on cartels.

Ciudad Juarez is now the most crucial battleground of a war that killed more than 6,000 people across Mexico last year and is scaring off investors in cities near the border.

"The solution is with the military. The federal, state and municipal police are infiltrated by organized crime," Valencia told Reuters.

The army expects to have 7,500 soldiers and federal police stationed in Ciudad Juarez by the end of the week, with a further 2,000 troops in the rest of Chihuahua state. Six local bishops pleaded in newspaper ads this week for an end to the killings that are "staining the state with blood".

Troops rolled past U.S.-style shopping malls in Ciudad Juarez on Tuesday to set up checkpoints at bridges running over the border and at the city's international airport, briefly shut last week after bomb threats.

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Calderon has about 45,000 soldiers across Mexico fighting cartels but has never before sent so many troops to one city.

At least four main cartels are fighting for control of Ciudad Juarez, and gangs of unemployed youths have joined the fray to extort businesses, kidnap residents, rob banks and work as hitmen.

Residents fear the city could go the way of Colombia's Medellin at the height of the drug war there in the 1990s, when murder rates hit 6,000 deaths a year.

"Juarez is prisoner to an infinity of groups fighting for the territory, and others who are making the most of the confusion for easy money," said army spokesman Enrique Torres.

Guzman, Mexico's most-wanted man, wants to seize Ciudad Juarez from local drug boss Vicente Carrillo Fuentes, and officials say the shadowy La Linea cartel from the western state of Michoacan and the feared Zeta hitmen from the Gulf of Mexico cartel are also at war here.

The jump in killings in the city to around 10 a day in February has put Calderon under intense pressure.

Some U.S. officials have publicly asked whether Mexico is becoming a failed state and voiced concern about a spillover of executions, kidnappings and extortions into the United States.

Last month, gunmen killed two city councilmen and forced out Ciudad Juarez's police chief by killing his deputy and vowing to murder an officer every 48 hours until he stepped down.

A former soldier attacked a convoy carrying Chihuahua's state governor in what many believe was an attack linked to drugs. Spooked by a series of death threats, the city's mayor now lives over the border in El Paso.

Ciudad Juarez, which boomed in the U.S. Prohibition era and now bulges with factories making goods for export, has pockets of normality during the day. Cars cram its shabby streets, residents sit in parks or walk their children to school.

But at night, the city once famed for its sex and tequila-fueled party life is ghostlike and residents adopt a self-imposed curfew from dusk till dawn.

"The drug hitmen are in control here. Things are out of control, there's so much death," said textile salesman Valente Salazar in Ciudad Juarez's main square as troops swept past in Humvees. "At six o'clock I go home and I don't go out at all after that. There are so many killings."

Take a look at the VIDEO: Drugs violence in Mexico claims more victims