The Mexican president, Felipe Calderón, has unveiled a "No More Weapons" advertising board, made using crushed firearms, near the US border and urged the US to stop the flow of weapons into Mexico.

The board, which is in English and weighs three tonnes, stands near an international bridge in Ciudad Juarez and can be seen from the US.

Calderón said its letters were made with weapons seized by local, state and federal authorities.

"Dear friends of the United States, Mexico needs your help to stop this terrible violence that we're suffering," he said in English during the unveiling ceremony. "The best way to do this is to stop the flow of automatic weapons into Mexico."

Before unveiling the billboard, the president supervised the destruction of more than 7,500 automatic rifles and handguns at a military base in Ciudad Juarez.

He said more than 140,000 weapons had been seized since December 2006, when he launched a crackdown against drug traffickers. More than 47,500 people have been killed since then.

Ciudad Juarez, where more than 9,000 people have died in drug-related violence since 2008, is one of the cities most affected by the violence.

The Mexican government said a federal prosecutor assigned to a northern state had been detained on suspicion of protecting the brutal Zetas drug cartel. The attorney general, Marisela Morales, said the federal prosecutor, Claudia Gonzalez, had been sent to prison. She did not say when Gonzalez was detained or give any further details.

Gonzalez was based in the city of Saltillo, the capital of the border state of Coahuila. The state, which borders Texas, has seen a spike of violence as the Zetas and Sinaloa drug cartels fight for control of smuggling routes into the US.