Bush signed the bill to build 1,100 kilometers (700 miles) of fencing, approved by the U.S. Senate last month, despite pleas from Mexico for a veto.

Fox told reporters in the Caribbean resort city of Cancún that the fence would not stop millions of Mexicans from heading north in search of jobs.

"It is an embarrassment for the United States," Fox said. "It is proof, perhaps, that the United States does not see immigration as a subject that corresponds to both countries."

Fox also said the U.S. plan "goes completely against a nation that prizes itself on being open" and which in the past has struggled to dismantle other barriers, such as the Berlin Wall.

President-elect Felipe Calderón, who takes over from Fox on Dec. 1, agreed.

"The decision made by Congress and the U.S. government is deplorable," Calderón said while on tour in Canada. "Humanity committed a grave error by constructing the Berlin Wall and I am sure that today the United States is committing a grave error in constructing a wall along our northern border."

Foreign Relations Secretary Luis Ernesto Derbez told W Radio that today is "a day of frustration."

The Foreign Relations Secretariat (SRE) later said in a news release that the construction of a wall affects the United States´ relationship with Mexico, as well as with Central American countries, from which thousands emigrate northward.

"It´s highly worrisome that the comprehensive approach, for which both administrations have fought ... is linked to a vision that exclusively considers security measures," said the statement.

The United States, Fox said, "cannot explain, in any way, its success and being the world´s greatest economic power if not for the immigration flows (into it) since the founding of that nation."

"It was a decision without a vision of the future, since it will also greatly affect (U.S.) society as it affects us," he said, adding that the plan was "useless" while also promising "to keep defending" the rights of Mexican emigrants in the United States.

The SRE news release repeated Mexico´s oft-stated position that solving the problem of illegal immigration "requires the establishment of new mechanisms that permit immigration that is legal, secure, ordered, dignified and respectful of human rights."

The speaker of the Chamber of Deputies, Jorge Zermeño, also rejected Washington´s plan.

"It´s a stance rejected by those of us who want relations to be conducted with respect, and besides, the decision does not resolve the problem. It´s money poorly-spent by the United States," emphasized the lawmaker, who belongs to the ruling National Action Party, as do Fox and Calderón.

However, Zermeño expressed confidence that relations with the United States would improve, and he said President Fox had achieved a victory by placing "for the first time in many years" the immigration issue "into discussion" between the two nations.