Mexico City  - It started with small groups of students demonstrating against the Mexican Army. A few days later the protestors were 200-strong and growing increasingly aggressive.

Officials in Mexico claim that powerful drug cartels are paying protestors to block streets in protest and disrupt life in the northern Mexican city of Monterrey.

In exchange for demonstrating against the military presence, recruiters are apparently handing out school backpacks and 500 pesos (about 35 dollars) in cash per person, the Mexican daily Reforma reported.

The cash is considered a tiny fortune in a country where unemployment is rising and the minimum daily wage is 3.60 dollars.

In Reynosa, a city on the border with the United States in the state of Tamaulipas, youth, women and children have blocked streets and border bridges. Similar incidents took place last year in Nuevo Laredo, Ciudad Juarez, Culiacan and Mazatlan.

These are cities with a significant army presence, especially after Mexico President Felipe Calderon decided in December 2006 to send in tens of thousands of military officers to fight the drug gangs.

For Jorge Luis Sierra, a Mexican specialist on military and security issues, these protests reveal the "need to show a much more intelligent response to the problem" of drug trafficking, since the drug gangs are flush with funds and can easily buy over the local people.

In Monterrey, the demonstrations that started last week slowly grew in aggression and violence. On Thursday, a police commander was killed in an incident that the governor of Nuevo Leon, Jose Natividad Gonzalez Paras, linked to the street blockades.

"Organized crime groups, part of a national network, decided to seize control of the main routes of communication, using common people as they have done in other cities in the country," Gonzalez Paras said at the funeral service for the officer.

"These organized crime groups have been threatening our security corps, local authorities, they have attacked the media, and on Wednesday we received a warning of reprisals," he said.

A few years ago, the National Defence Ministry warned that the drug cartels were financing campaigns to damage the reputation of the army. However, some protests had also been linked to instances of excessive use of force by the military.

"The authorities have to be extremely sensitive to distinguish very clearly which protests are legitimate and which protests are not," Sierra explained.

From Houston, Texas, where he lives, the security expert told Deutsche Presse-Agentur dpa over telephone that "there is a mix between people who are protesting legitimately and people who are not protesting that way."

The military estimates that some 400,000 people in Mexico have direct or indirect ties with drug trafficking, Sierra noted.

"In the face of that there are many responses, short-, medium- and long-term," he said.

Firstly, greater intelligence work is required to identify the links between the leaders of these demonstrations and organized crime groups. But the solution to the problem isn't that simple.

"In the long run what one will have to do is to create more opportunities for people, because many people who establish links with drug trafficking do so because they have no other source of income," Sierra said. (dpa)