Relatives of 43 missing Mexican students, who the authorities say were murdered by a drugs gang, have led mass protests in the capital.
The marches were largely peaceful but some protesters clashed with police near the presidential palace.
The BBC reports convoys carrying the families arrived in Mexico City on Thursday after touring the country to rally support.
Many remain unconvinced by the official explanation for the students' disappearance and hope they are alive.
Masked protesters clashed with police near the airport hours before the three marches started.
Francisco Lagro, father of 19-year-old Magdaleno, one of those missing, was travelling on one of the caravans towards the capital on Thursday.
"It's been almost two months without knowing where they are. We don't know anything and we're desperate," he said.
"What are they doing? In what conditions? Do they get any water or food? Are they tied up? We have so many questions."
Corruption and political violence are endemic in Mexico. In the last decade, 100,000 people have died and 27,000 people have gone missing.
Yet this single incident, the disappearance of 43 students in the southern state of Guerrero, has galvanised all of this opposition here in the centre of Mexico city.
Thousands of people took part in three protest marches in the capital, which started at 17:00 local time.
Many thousands converged on Mexico City's main square, or Zocalo.
Several hundred protesters gathered near the presidential palace, where police tried to push them back using water cannon. There are no reports of any injuries.
In the violence near Mexico City's international airport earlier on Thursday, some 200 hooded protesters threw rocks and Molotov cocktails at police officers who had been trying to disperse them.
Many shops and businesses were reportedly closed because of the marches.
Demonstrators have also called for a nationwide strike. Protests also took place in other parts of Mexico and abroad.