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Cuernavaca, Mexico (CNN) -- Hundreds of protesters began a three-day march to Mexico's capital Thursday, demanding peace in the war between the government and drug cartels.

Some carried signs bearing the names of victims of the brutal wave of drug-related violence that has hit many parts of the country. Others who gathered in the central Mexican city of Cuernavaca toted a large black banner that said "STOP THE WAR."

Poet Javier Sicilia held a Mexican flag as he began the 80-kilometer (50-mile) walk, but he sharply criticized the country's president, who began a crackdown on drug cartels shortly after taking office in December 2006.

"We cannot understand a war that is badly planned, a war that is badly directed. We cannot understand why he does not understand why the criminals are out there. If they are out there, it is because the institutions and the state are co-opted," Sicilia said.

The well-known writer has become one of the most outspoken opponents of Mexico's drug war and widespread drug-related violence since his son's slaying in March. He has led several demonstrations and vocally criticized officials' handling of the case.

Thursday's march is scheduled to culminate Sunday with a large demonstration in Mexico City's central square. The goal, Sicilia said, is to call for an end to violence and demand that Mexican authorities sign an agreement to re-establish peace and justice in the country.

Without referring directly to the marchers, Mexican President Felipe Calderon issued a statement late Wednesday night reiterating his government's commitment to fighting organized crime.

"Retreating from the fight is not an option. Quite the opposite. We must redouble our efforts, because if we stop fighting, they are going to kidnap, extort and kill all over the country," Calderon said. "Because marching back means things will get worse. If we retreat, we will allow gangs of criminals to walk all the streets of Mexico with impunity, assaulting people without anyone stopping them."

Sicilia's 24-year-old son was found dead on March 28, crammed into a car with six other bodies in Cuernavaca. Masking tape was wrapped around their head, faces, wrists and ankles.

Authorities believe all seven victims suffocated to death, and they have said members of Mexico's Pacifico Sur cartel are responsible. Two suspects have been arrested.

The case of Sicilia's son caught Calderon's attention, and the interest of the Mexican public, well before Thursday's march.

In an interview with the Mexican newspaper Excelsior last month, Calderon said the government is committed to finding those responsible for the crime.

"We already have clearly identified those responsible. ... We are on their trail and I hope we can capture them," he said.

Sicilia has said his crusade to demand justice is not just about his son.

"There are many dead and there is much pain. ... These citizens have the same dignity as my son," he told CNN in a recent interview.

In his statement Wednesday, Calderon acknowledged that some Mexicans are less committed to -- and afraid of -- his fight against criminals. But he showed no sign of changing his approach.