MEXICO CITY, Mexico (AP) -- Thousands of students, union leaders and radical activists flooded the streets of Mexico's capital Saturday to mark the anniversary of the October 2, 1968, massacre of protesters in Tlatelolco plaza.

Unlike in years past, the 36th-anniversary demonstrations were largely peaceful, as protesters gathered in Tlatelolco, then marched 1.5 miles (2.2 kilometers) to the main square of Mexico City's historic downtown district.

In the only major incident, about 60 anarchists clashed with police before the main march begun. There were no reports of arrests or serious injuries, however.

Protesters carried banners and waved signs demanding President Vicente Fox bring to justice those responsible for the massacre, in which snipers and army troops fired on a pro-democracy student demonstration that came 10 days before Mexico City began hosting the Olympics.

Most businesses along the route closed and lowered protective metal shutters before the march began, and those which remained open used bulky metal slabs to reinforce storefront windows.

More than 3,500 police officers took to the streets along the march route to keep order. At least six people were arrested, many for carrying open containers of alcohol, authorities said.

The annual demonstration has turned violent in recent years, and in 2003 radical demonstrations inspired by the main march saw protesters smash store windows, batter cars and blanket buildings with spray paint. Last year's events prompted police to arrest 75 people -- including 50 youths under age 18 -- on suspicion of robbery, assault and property damage.

While the government has never released firm figures on those killed at Tlatelolco, estimates range from 38 to several hundred people.

In a statement released Friday, the office of a special prosecutor appointed by the president said it was nearly finished recording the testimonies of hundreds of people with information about the mass-killing as part of its continuing effort to seek arrest warrants for some of those responsible.

Once-secret government files indicate the killings began when snipers under the command of the Mexican government fired into the crowd. At least 360 government gunmen were present, the documents said.

Government officials at the time said armed dissidents provoked the deadly clash by firing on police during the protest.

At his daily press briefing, populist Mexico City Mayor Andres Manuel Lopez Obrador said there was "no doubt" state forces supported by soldiers did the shooting, and added that those responsible should be punished.

He ordered the capital's flags flown at half-mast to mark the anniversary of the massacre.

Copyright 2004 The Associated Press.