Reacting to the Bush administration's planned deployment Mexico said on Tuesday it would file lawsuits in US courts if National Guard troops become directly involved in detaining immigrants.
"If there is a real wave of rights abuses, if we see the National Guard starting to directly participate in detaining people ... we would immediately start filing lawsuits through our consulates," said Luis Ernesto Derbez, the Mexican foreign secretary.
George Bush, the US president, announced on Monday that he would send 6,000 National Guard troops to the 3,200-km border.
But he said the troops will provide intelligence and surveillance support to US Border Patrol agents and will not catch and detain illegal immigrants.
On Tuesday, Mexican newspapers characterised the decision as a hardening of the US position.
"If there is a real wave of rights abuses, if we see the National Guard starting to directly participate in detaining people ... we would immediately start filing lawsuits through our consulates"
Luis Ernesto Derbez,
the Mexican foreign secretary
Some criticised Vicente Fox, Mexico's president, for not taking a stronger stand against the measure, although Fox called Bush on Sunday to express his concerns.
Fox's spokesman, Ruben Aguilar, said that Mexico accepted Bush's statement that the Guard troops did not imply a militarisation of the area, and that Mexico remained "optimistic" that the US Senate would approve an immigration reform "in the interests of both countries".
He noted that Bush had expressed support for the legalisation of some immigrants and the implementation of a guest worker programme.
"This is definitely not a militarisation," said Aguilar, who also dismissed as "absolutely false" rumours that Mexico would send its own troops to the border in response.
But Felipe Calderon, a presidential hopeful belonging to Fox's National Action Party, issued a statement that the military presence would endanger migrants without stopping them.
"These measures have been proven mistaken. They increase the social and human costs for migrants and only benefit criminal groups that make money on the hopes and suffering of those looking for an opportunity," he said.
Criticised at home
In the United States, opposition Democrats said sending thousands of troops to the Mexican border would strain US forces already under pressure because of operations in Iraq and Afghanistan.
"Putting another burden on the backs of men and women who are serving their second tours of duty in Iraq and Afghanistan isn't the right answer"
John Kerry, the beaten Democratic candidate in the 2004 presidential election, said tightening border security was needed.
But, he said, "putting another burden on the backs of men and women who are serving their second tours of duty in Iraq and Afghanistan isn't the right answer".
And some in Bush's own Republican Party said they feared the president was not being tough enough on illegal immigration, a topic which has sparked a fierce political debate ahead of US congressional elections in November.
"A few weeks ago, the administration announced a crackdown on illegal employers, arresting more than 1,100 nationwide. But by the next morning, more than half of those arrested were released," said a Republican congressman, Tom Tancredo.
Tancredo said he would be watching and waiting to see if the president follows through on his pledge to enforce the law and crack down on employers of illegal immigrants.