Eight mass graves filled with corpses were uncovered in La Joya, a farming village in the San Fernando township, authorities said.
They fear the number of dead will rise as remains have only been counted in three of the eight pits.
Police and military staff learned on March 25 that several buses had disappeared in the area, leading to their investigation which turned up the grisly find.
A military patrol located the mass graves. The Tamaulipas state prosecutor’s office said 11 people had been arrested and another five kidnapping victims had also been set free.
“We are trying to establish if the remains are those of the people who went missing on the buses,” the prosecutor’s statement said.
Thousands of outraged citizens took to the streets of 38 Mexican cities. Crowds swelled to about 10,000 in Mexico City.
The protest marches were organised following the murder of a well-known author’s son along with four close friends and two others on March 28.
Javier Sicilia, a poet and columnist for the daily La Jornada and the weekly Proceso — two of the country’s leading publications — called for the protests following the killing of his son Juan Francisco.
“We are tired of this war that is not ours,” said Sicilia asking the government to “stop this senseless war that has mostly killed civilians.”
Seven major drug gangs are operating in Mexico whose bloody clashes have left over 34,600 people dead since December 2006, when President Felipe Calderon’s government launched a military crackdown that has so far failed to put a damper on the violence.
Over 3,000 people were killed this year alone, according to figures cited by the Mexican media.