Protests in more than 20 Mexican cities against drug-related violence have been interrupted by news of the discovery of 59 bodies.
The bodies were found in Tamaulipas state by police investigating the abduction of bus passengers.
The area is a hotspot in the violence related to drug trafficking that is ravaging parts of Mexico.
The violence was condemned by protesters in the main square in Mexico City, who chanted: "No more blood!"
Some called for President Felipe Calderon to resign, saying his strategy had exacerbated the bloodshed.
About 35,000 Mexicans have died in drug-related violence since the president began deploying the army to fight the cartels in December 2006.
The demonstrations were inspired by poet and journalist Javier Sicilia, whose son was killed last week.
Mr Sicilia has blamed Mexican politicians as well as criminal gangs for the violence, saying they have "torn apart the fabric of the nation".
Small demonstrations were also held in New York, Buenos Aires, Paris, Madrid and other cities around the world.
The bodies were found in eight separate graves in the area of San Fernando, police said - with 43 corpses found in a single grave.
Authorities in Tamaulipas said they had found the graves while investigating reports that, late last month, buses in the area had been stopped and passengers pulled off and kidnapped .
In a raid, 11 people were arrested, while five others being held captive by the alleged kidnappers were freed.
A state minister told the Associated Press news agency two of the dead were women, and many of the victims appeared to have died between 10 and 15 days ago - dates that would roughly match the bus abductions.
It appears to be the largest single discovery of bodies since 72 Central and South American migrants were found dead in the same town last August, after drug gangs killed them for refusing to work for them.
After that incident, President Calderon deployed more troops to fight criminals who use the area as a route to traffic narcotics into the US. And our correspondent in Mexico City, Julian Miglierini, says this most recent discovery will come as an embarrassment for him.
The Mexican government says it is making progress against the drug cartels, and has captured or killed many of their top leaders.
It says much of the bloodshed is the result of fighting between rival criminal gangs.
This view was echoed by the head of the US Drug Enforcement Administration, Michele Leonhart, at an international conference in the Mexican City of Cancun on Wednesday.
"It may seem contradictory, but the unfortunate level of violence is a sign of success in the fight against drugs," the DEA chief said.
The cartels "are like caged animals, attacking one another," she added.
Wednesday's protests were called by Mr Sicilia after his 24-year-old son, Juan Francisco, was found dead inside a car along with six other people in the city of Cuernavaca last week.
In an open letter to Mexico's politicians and criminals published in Proceso, he said President Calderon's campaign against the drugs gangs was "badly planned, badly carried out and badly led".
"The citizenry has lost confidence in its governors, its police, its army, and is afraid and in pain".
Mr Sicilia also condemned the criminals as "subhuman, demonic and imbecilic".
"We have had it up to here with your violence, your loss of honour, your cruelty and senselessness," he wrote.
Before joining the demonstrations, Mr Sicilia met President Calderon in Mexico City.
He said the president offered his condolences and briefed him on efforts to find his son's killers.