Mexico's President Felipe Calderon speaks during a news conference in Mexico City, Monday. Earlier in the day, Calderon condemned the new Arizona new immigration law as discriminatory. Mexico has issued a travel warning to Mexican citizens in Arizona.

Americans are accustomed to State Department advisories cautioning them to steer clear of dicey protests and political turmoil in developing countries, including their neighbor to the south.

Since drug violence erupted in Mexico, the US has warned its citizens of “large fire fights” in towns across Mexico, particularly along the US-Mexico border.

Mexico always grumbles about US travel alerts. But today Mexico got payback.

IN PICTURES: The US/Mexico border

In big red letters on its “travel guide” on the exterior ministry website is a travel alert for “all Mexicans visiting, living, or studying in the state of Arizona.”

Since Arizona signed a law that requires people suspected of being illegal immigrants to show proof of legal residence when asked by law enforcement, the Mexican government is warning citizens of the “adverse political atmosphere for migrant communities and for all Mexican visitors.”

When the law takes effect, foreigners without the proper identification can be detained, and even held in migration detention centers, it warns. Until details of how the law will be implemented are clear, it reads, “all Mexican citizens could be bothered or questioned without motive at any moment.”

It also warns of protests that have erupted because of the move.

On Monday, Mexican President Felipe Calderón condemned Arizona for a move that he says “opens the door to intolerance, hate, discrimination, and abuse in law enforcement,” he said.

He promised to help Mexican citizens – whether in the US illegally or not – deal with any violations of their rights.

The government’s travel warning lists contact information for all consulates in the state of Arizona and provides a free, 24-hour help line for all Mexicans residing in Arizona.

The last warning Mexico’s exterior ministry posted, according to the website, was in July 2009, alerting travelers that Canada began requiring Mexicans to obtain visas to visit.

In practical terms, today’s warning will probably not deter those Mexicans who travel frequently to the US to carry out business, visit relatives, and, for the most well-heeled, go on shopping trips. All Mexicans need visas to board a plane north, no matter where they are heading.

It might, however, change the fluid migration routes into the US, which adapt depending on what stretch of the border is more fortified and what kind of local laws are in place.

And President Calderón, who is under pressure from groups to take a firm and angry stance against the Arizona decision, will likely win a political point for this one.

He has warned that trade and political ties will be “seriously affected,” he said. So far, though, he has not specified how.

IN PICTURES: The US/Mexico border