The mass killing, mass graves and mass arrests of police this week in northeastern Mexico have shaken the government of President Felipe Calderon, who faces mounting pressure to stem sensational violence.

Sixteen police officers from the town of San Fernando have been detained on suspicion of protecting the Zetas criminal gang, which is alleged to have filled mass graves with dozens of bodies found there, according to Mexican officials who announced the arrests Wednesday night.

In yet another case of alleged police complicity in murder, authorities found the bodies of four men detained three weeks ago in the border city of Ciudad Juarez. The men disappeared in the system after they were arrested by a special forces unit of municipal police. Their bodies, showing signs of torture, turned up in the high grass at a ranch Wednesday.

Four years into his U.S.-backed, military-led war against drug cartels, Calderon’s government is struggling to make good on promises to transform the state and local police forces, whose officers are often ill trained and poorly paid. These same units often work for crime mafias and drug traffickers. Calderon’s opponents in the Mexican congress have blocked his efforts to place local police under state control.

Authorities searching shallow pits in San Fernando, about 90 miles south of Brownsville, Tex., said they found 23 more corpses Thursday, bringing the total to 145 since the first were uncovered last week.

On Thursday, investigators began moving the overflow to morgues in Mexico City to help with identifications.

Locals have dubbed the main artery between the Tamaulipas state capital, Ciudad Victoria, and Brownsville as “the highway of death.” Bus companies have canceled travel, and the few Mexican journalists who have ventured into San Fernando arrived with military escorts.

Investigators suspect that some of the victims were pulled off buses by gunmen who had set up roadblocks. In a video clip by the newspaper El Universal, witnesses describe women being dragged away and raped.

Travelers are advised to drive a circuitous route hundreds of miles to the west to the reach the border.

Responding to protesters who have criticized the government for failing to reduce the carnage, Calderon has called on citizens to rally together under the banner “Ya basta!” — roughly “enough is enough.”

The president warned that his critics should be attacking the true source of violence, the criminals, and not the government or armed forces.

The response from opponents was lukewarm. Jorge Carlos Ramirez Marin, a federal deputy and leader of the opposition Institutional Revolutionary Party, said, “Enough is enough — this strategy isn’t working and must be changed. The words of the president sound nice, but they are useless.”