Violent clashes between rioters and police have marred the end of a summit of EU and Latin American leaders in Mexico's second city, Guadalajara.
The rioters did not appear to be promoting a particular cause
French President Jacques Chirac was forced to cancel a news conference as well-organised groups of protestors pelted riot police with missiles.
Around 20 people were injured and more than 90 arrested amid the mayhem just a short distance from the venue.
The authorities say a violent student movement was behind the anarchy.
There appeared to be a group of about 50 rioters who struck during a peaceful rally by hundreds of people in support of fair trade and against poverty.
The situation quickly deteriorated and the police responded to the rioters with equal force, hitting protesters with their batons.
They fired tear gas and used water cannon to try to control the situation but it took several hours for them to break up the crowds.
Despite the huge security operation which had been put in place to protect the conference delegates, terrified passers-by were caught up in the aggression.
Dozens of civilians along with a number of police were injured.
Banks and shops across the city have been looted.
Debris is now strewn across the streets.
The BBC's Claire Marshall reports that, at the end of a high-profile summit, this is a real embarrassment for Mexico.
The summit reached only limited agreement on trade reform, with the EU giving a guarded welcome to a proposal by the G20 - a group of developing states led by India, South Africa and Brazil - for agricultural tariff cuts.
On foreign policy, however, the 58 leaders united to urge the US to seek greater UN involvement in Iraq and "express abhorrence at recent evidence of the
mistreatment of prisoners in Iraqi prisons".