Mexico protesters clash with police

"We have 18 police officers wounded, not seriously," Manuel Mondragon, a police chief, said.

"Twenty people were arrested, all precisely identified by video surveillance systems," Jose Angel Avila, secretary of Mexico City municipality, said.

Call for justice

The clashes were sparked near Zocalo square when police tried to arrest protesters who had painted graffiti on a city building wall, a witness said.

The demonstrators were marking the anniversary of a massacre in Tlatelolco, when Mexican security forces opened fire on an 8,000-strong political protest.

Between 44 and 300 pro-democracy activists are believed to have died in the police attack.

The Mexican police covered up evidence of the atrocities, which came 10 days before the 1968 summer Olympic games.

"I'm here to denounce the most despicable act committed in Mexico," Aarceli Bernal, a 26-year-old student, said.

Protesters in Tlatelolco drew chalk figures on the ground covered with blood stains to remember those killed by the security forces.