The military fragmentation grenades shattered a family-friendly gathering of thousands in the cobblestone streets not far from where President Felipe Calderon grew up. He pledged an immediate military response Tuesday and urged Mexicans not to be afraid.
However, in a country increasingly terrified and outraged by both drug violence and common crime, the attack drove home a tragic message: No place is safe.
"These illegal acts were clearly attacking our national security, committed by true traitors who have no respect for others or for the country," Calderon said. "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 sent more than 25,000 soldiers to confront the cartels that move marijuana and cocaine into the United States, and the gangs have responded with daytime shootouts, assassinations, beheadings and massacres. Michoacan state, which is home to Morelia, has seen more than its share of this violence.
The latest came during the traditional "grito," or shout for independence, late Monday night. Michoacan Gov. Leonel Godoy had just finished shouting "Viva Mexico!" from a balcony, when the two grenades exploded simultaneously in the crowd, blocks apart.
At first, the throngs of families thought the explosions were part of the fireworks display. Then thick, black smoke rose from the crowd, people started screaming and the cathedral's bells fell silent. As the crowd cleared, rescuers attended to the wounded and dead.
Both state emergency officials and state prosecutors said seven were killed, although there were earlier reports that the death toll had risen to eight.
Godoy, who was unhurt, said witnesses saw a heavyset man wearing black throw one of the grenades, then beg forgiveness for what he had done. But he provided no more details, and there were no immediate claims of responsibility. Authorities made no arrests.
"Without a doubt, we believe this was done by organized crime," he said. The governor's office later said he was traveling to Mexico City late Tuesday to meet with Calderon.
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 parades, held in cities and towns across the nation to celebrate the 1810 start of Mexico's 10-year war of independence from Spain.
Godoy canceled Tuesday's march in Morelia after his office received threats, "because there are children, women and innocent people who have been hurt." But parades went on elsewhere, including the traditional military show of force in Mexico City.
Eunice Arevalo, 23, attended that parade with 10 other family members, but she said she was fearful for 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. "So far 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."
Calderon urged Mexicans to stand up to the cartel threat, appealing to their patriotism.
"The Mexican people, especially on this important date, should remain united in the face of those who want to divide us," the president said.
A week after nationwide marches designed to symbolically retake the streets from criminals, many echoed his 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.
Morelia remained under heavy guard, with soldiers and federal, state and local police manning checkpoints on surrounding highways. Seventy-five injured were hospitalized, civil protection officials said.
Meanwhile, families mourned the dead. Isabel Sanchez said her brother Alfredo Sanchez was about to retire as a metalworker after surviving a stroke. He was killed amid the crowd as his wife parked the car.
"My brother was fighting to live," she said. "We are tired of living like this. ... I don't understand why my brother died."
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.