MEXICO CITY, April 3 (Reuters) - Mexico's human rights commission criticized the army on Thursday for robbing police officers and stripping off their clothes before interrogating them in a crackdown on the drug trade near the U.S. border.

The commission said it was investigating the arrests by the army of 21 police officers in the border city of Ciudad Juarez on Tuesday who were held for more than a day without charge.

During that time, troops removed female officers' clothing and swiped cash and credit cards from the officers before interrogating them, the commission said in a statement.

"Personnel from the Human Rights Commission of Mexico are carrying out several investigations to ... identify and prove possible cases of torture," the commission said.

Some 1,500 soldiers were sent to Ciudad Juarez, just across the border from El Paso, Texas, in recent days to clamp down on violence between warring drug gangs.

Poorly paid police have often been accused of taking sides in the fight between the local Gulf Cartel and the rival Sinaloa gang that is moving into the area.

President Felipe Calderon has sent 25,000 soldiers and police to smuggling hot spots since taking office in December 2006. Ciudad Juarez previously had a very light military presence. (Reporting by Jason Lange; editing by Mohammad Zargham)

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