By Miguel Angel Gutierrez

MEXICO CITY, May 16 (Reuters) - Opposition Mexican lawmakers demanded on Wednesday that soldiers be taken off a joint police and army crackdown on drug trafficking gangs, following a report of rights abuses by some troops.

In a challenge to conservative President Felipe Calderon, lawmakers from the Party of the Democratic Revolution or PRD, and the Institutional Revolutionary Party, PRI, told a committee of senators and deputies that troops deployed in the 5-month-old offensive should be sent back to their barracks.

"The army should not be used indiscriminately or in a permanent way in the fight against drug trafficking," said Carlos Navarrete, leader of the PRD in the Senate upper house.

"Arguments, reasons and unfortunate facts are piling up to show that the first negative consequences of the massive and permanent use of the army in fighting organized crime are already appearing," he said.

The drug violence gripping Mexico is by far the biggest challenge facing Calderon, who took office in December.

He won plaudits in Washington by immediately sending thousands of troops and federal police out to northern and western Mexico to combat feuding between rival cartels that is killing some half a dozen people a day.

But a report by Mexico's human rights ombudsman Jose Luis Soberanes said some soldiers had been making arbitrary arrests and sexually abusing minors.

The report listed more than 50 complaints against the army, mainly in the western state of Michoacan, that included sex abuse of underage girls, torture and illegal searches.

"The reports of missing people, torture, bad communication, arbitrary detentions and illegal searches are a clear example of what could happen in many parts of the country," PRD lawmaker Jesus Zazueta told Wednesday's meeting.

VIOLENCE UNABATED

In Mexico, some have applauded the presence of military checkpoints and soldiers on the streets in areas under the control of brutal drug gangs -- but others say the job should be left to the police.

Despite the extra firepower, violence has continued unabated from last year, when more than 2,000 people died in gangland killings across the country. With the new crackdown, hitmen are increasingly targeting police and army chiefs.

After the report of rights abuses, PRI deputy Marco Antonio Bernal demanded an investigation, and the defense ministry said soldiers would be punished in any cases that were proved.

Being in the line of fire of cartel hitmen marks a drastic change for Mexico's army, which is more used to low-risk operations like protecting communities from hurricanes.

Some 25,000 soldiers and federal police are deployed in the crackdown, which began in Calderon's home state of Michoacan.

Deputy Attorney General Noe Ramirez said recently that Mexico's police forces were too understaffed to be able to spread out across the country in the way the army could.

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