Suspected cartel thugs threw two grenades into a crowd of thousands of Independence Day revelers, killing eight and injuring more than 100 in a terrorist attack that seriously ramps up Mexico's drug war.

The attack was, for Mexico, the rough equivalent of a domestic terrorist hit on a July Fourth parade in Texas, and President Felipe Calderon acted quickly on Tuesday. In a nationally televised address, he urged citizens not to be afraid, and pledged an immediate military response.

The two military fragmentation grenades exploded simultaneously, about 500 yards (meters) apart, during the traditional "grito," or shout for independence, in Morelia's main plaza late Monday night. It caused panic among throngs of families who had gathered to celebrate in the colonial capital of Michoacan, Calderon's home state.

"These illegal acts were clearly attacking our national security, committed by true traitors who have no respect for others or for the country," said Calderon. "Those who believe they can use fear to hold our society hostage and immobilize us, are mistaken. ... They are doomed to fail."

Since taking office in 2006, Calderon has dispatched more than 25,000 soldiers to confront the cartels that bring marijuana and cocaine into the United States, and the cartels have responded fiercely, unleashing daytime shootouts, assassinations, beheadings and massacres. Michoacan has seen more than its share of this violence.

Michoacan Gov. Leonel Godoy, who was leading the ceremonies, was unhurt. He said witnesses saw a heavyset man in black throw one of the grenades and beg forgiveness for what he had done. But he provided no more details, and there were no immediate claims of responsibility. "Without a doubt, we believe this was done by organized crime," he said.

The attack targeted a cherished tradition that brings millions of Mexicans together in public plazas each year, and cast an immediate pall over Tuesday's Sept. 16 parades, held in cities and towns across the nation to celebrate the 1810 start of Mexico's 10-year war of independence from Spain.

The violence didn't stop Eunice Arevalo, 23, from attending Mexico City's parade with 10 other family members. But she and others were fearful about Mexico's future.

"This is not going to stop. This is only going to get worse," said Arevalo, a cooking student whose father is a soldier. "Up until now the killers have targeted other drug traffickers, but now it seems we're going to see still more violent acts against everyday citizens, just to shock people."

Others expressed defiance.

"Mexico is ours. We won't hide. We are going to go out and take back our streets," said lawyer Juan Enrique Arguijo, 46.

Calderon also appealed to Mexicans' patriotism against the cartel threat.

"The Mexican people, especially on this important date, should remain united in the face of those who want to divide us," the president said.

Godoy canceled Tuesday's march in Morelia after his office received threats, "because there are children, women and innocent people who have been hurt."

Morelia remained under heavy guard, with soldiers and federal, state and local police manning checkpoints on surrounding highways. Authorities have made no arrests.

The state prosecutor's office identified the explosives as fragmentation grenades, illegal in Mexico. Civil protection officials said the death toll rose to eight people on Tuesday, and 75 of the injured were hospitalized.

The attack comes only days after 24 bodies were found bound and killed execution-style in a rural area outside Mexico City in one of the largest massacres in recent history.