MONTERREY, Mexico -- Mexicans have endured plenty of horrific crimes during their country's bloody five-year war against drug gangs: bodies hanging from overpasses, beheadings, mass slayings of migrants and gunfights on crowded streets.

The torching of the casino that killed at least 52 people Thursday, however, was a shocking new low for many.

In a televised speech Friday, President Felipe Calderón declared three days of mourning and called the attack on the Casino Royale in Monterrey the worst against civilians in the nation's recent history. "We are not confronting common criminals," he said. "We are facing true terrorists who have gone beyond all limits."

The attack was different from others in recent years in that the victims weren't cartel foot soldiers or migrants resisting forced recruitment by gangs. They were middle-class people, working or gambling in an affluent part of a city that was considered one of Mexico's safest.

"The media impact that this has is greater, because we're talking about an attack on a civilian population of a certain income," said Jorge Chabat, an expert in safety and drug trafficking at the Center for Research and Teaching in Economics. "Because who was there was from the middle class, the upper middle class of an important city in Mexico."

As the country took in the grisly details of the attack, some said a new, macabre milestone had been reached in a conflict that's claimed nearly 40,000 people since Calderón launched his drug offensive in December 2006. Calderón urged his people to unite against the cartels.

"Today, Mexico is upset and saddened, and we have to transform this sadness and this grief into courage and valor to face . . . these criminals," said Calderón, who did not say whether his government would alter its offensive against the cartels.

Calderón announced he is sending more federal forces to the city of 1 million people.

Hours later, he appeared in front of the burned-out casino and held a silent, minute-long vigil. "We don't know how to protect ourselves or whom we're talking to," he said. "We don't have security right now."

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