MEXICO CITY — Mexican federal police officers are mutinying over the government’s decision to make them part of the country’s new National Guard, a powerful challenge to President Andrés Manuel López Obrador as he tries to fight rising crime.

Earlier this year, faced with record-setting homicide rates, Mr. López Obrador announced he would combine the navy, army and federal police into a new security force to curb violence and fight organized crime.

Now just days after the new National Guard began to be officially deployed across the country as the centerpiece of Mr. López Obrador’s security plan, police officers have rebelled.

The officers say that their pay and benefits will be cut and that they could be fired if they refuse to join. They also say they face poor conditions on deployments away from home and worry that they will serve under military commanders who do not understand civilian policing.