Thousands of protesters blocked the highway to Acapulco in southern Mexico to press authorities to find 43 students missing since a deadly police shooting last weekend.
The demonstration was part of commemorations of a 1968 massacre of students by the army in Mexico City but the protesters focused on the fate of the disappeared in violence-plagued Guerrero state.
Parents of the missing led the march in the state capital Chilpancingo, holding signs saying "they were taken alive, we want them back," as a sea of students, teachers and others followed behind.
The students, from a teacher training college near Chilpancingo, had gone to the town of Iguala 100 kilometers (62 miles) to the north last Friday to raise funds when they were shot at by municipal police after they hijacked buses to return home.
Three students were killed and witnesses said they saw dozens taken away in police cars.
Authorities detained 22 officers over the shooting and another attack that killed three other people later that night on the outskirts of Iguala.
A gang is believed to have participated in the second shooting and the officers are suspected of having links to criminals in the region, raising fears a gang has the students.
"We are fed up with crime and corruption in this state," said Manuel Martinez, 32, whose 18-year-old nephew is among the missing.
Relatives of the missing, backed by scores of troops and state police, scoured fields and communities around Iguala this week, handing out flyers with pictures of their loved ones.
But the parents said they are unhappy with how authorities are handling the case, saying no proper investigations are being conducted for the search.
"The search was a show," said Mariano Flores Vazquez, 35, who searched for his 22-year-old nephew.
Manuel Olivares, coordinator of the Guerrerense Network of Human Rights Organizations, told AFP the parents are debating how to proceed with the search.
"The parents didn't like how it took place because there's no seriousness, there's no investigation," he said.
Officials say they hope the 43 students will turn up alive like a dozen others who reappeared after apparently going into hiding.
The parents are holding out hope but some fear for the worst in a country where 80,000 people have been killed in drug-related violence since 2006. Another 22,000 people are unaccounted for.
As the students marched, Governor Angel Aguirre sent 1,600 state workers from various departments to help the search in Iguala.
"We will go house to house, under orders of Governor Angel Aguirre, until we find these youngsters," said Beatriz Mojica Morga, the state's social development secretary.
The Iguala case emerged as the army grappled with its own scandal after eight soldiers were detained by the military over the killing of 22 drug suspects.
Authorities originally said the suspects died in a shootout but a witness said the soldiers executed 21 people, including her 15-year-old daughter, after a shootout.
While eight soldiers face military sanctions, the federal attorney general's office is charging three of them with homicide over the killings in Tlatlaya, a town south of Mexico City.
Thousands of people marched in Mexico City for the anniversary of the killings at Tlatelolco square, where soldiers killed dozens of student protesters just 10 days before the capital hosted the Olympic Games.
Protesters in the capital also denounced the Iguala and Tlatlaya cases, noting that like Tlatelolco, they took place under the long-ruling Institutional Revolutionary Party (PRI).
"The same story repeats itself. It doesn't surprise anybody that it's the same PRI. The same who killed dozens of students in cold blood in (Tlatelolco) is the same that shows indiferrence over the disappearance of the students in Guerrero," said Rosa Icela, a 41-year-old public worker.