Thousands of Mexicans dressed in white marched silently Sunday through the streets of Juarez, Mexico, just across the border from El Paso, Texas to protest drug-related violence. that killed more than 100 people, including about 20 police officers, during the past week in Mexico. VOA's Greg Flakus has been following the situation from our Houston bureau.

Ordinary citizens in Mexico are reacting with alarm as the war between rival drug smuggling groups erupts in city centers and residential neighborhoods. Law enforcement experts say drug gangs are reacting violently to efforts by police and the army to break up their operations.

Death threats sent to many police departments have provoked at least some police officials to resign. Those who do not resign often pay a heavy price for their devotion to duty.

Saturday, gunmen killed Municipal Police Chief Juan Antonio Roman in front of his house in Ciudad Juarez. He fired back at his assailants, but was outnumbered and outgunned. Investigators say his assassins fired more than 60 shots. His was the third murder of a senior police official in Juarez last week.

At least nine federal agents have been killed during the past two weeks as well. In Mexico City, on Thursday, gunmen killed Edgar Millan, commander of the Federal Preventive Police, one of the key agencies involved in the national offensive against organized crime. President Felipe Calderon started the war on crime shortly after he took office in December, 2006.

While some political observers and many citizens worry that the drug cartels may be winning the fight, Calderon is calling for his countrymen to stand firm against the criminals. He says society must unite with the government to repudiate the violence of criminal gangs, which he says are trying to terrorize the nation. Calderon says Mexico cannot surrender any part of its territory to these violent criminals.

The Mexican leader has put about 30,000 armed troops into the field, mostly in border areas.

There is little indication that this has slowed the flow of cocaine, marijuana, heroin and other drugs across the U.S. border, but the capture of some important gang members has left gaps that other criminals have been only too eager to fill. By some counts, the resulting battle over the lucrative trade routes has led to as many as 3,000 deaths.

While President Bush has praised Calderon's offensive against the drug-trafficking gangs, some Mexican political analysts and human rights groups question the effectiveness of the strategy. One area of concern is the use of soldiers to carry out police work, which human rights groups say can lead to abuses.

But some crime experts say the military is needed because there is so much corruption within Mexico's law enforcement agencies and local police forces. Investigators suspect the murderers of Edgar Millan, for example, were aided by someone within his own agency. Most rank-and-file police officers in Mexico are underpaid, under-trained and under-supported in equipment and arms.

President Bush has asked Congress to help Mexico with its fight against the criminal gangs by approving a $500 million aid package that includes helicopters and other equipment, and also training for Mexican forces on how to fight smuggling and corruption in police forces.