Demonstrators have smashed shop windows and set fire to cars in central Geneva during a protest against the World Trade Organisation (WTO).
Witnesses said police in riot gear moved to head off a group of demonstrators intent on vandalism in the city's main shopping street. More than 30 people were arrested.
Geneva Police Chief Monica Bonfati said officers arrested four looters in addition to the protesters. In all, 33 people were detained. No injuries to police or protesters were reported, but an 80-year-old woman passing by was knocked to the ground and hurt.
The organisers of the protest march, whose route had been previously agreed with the authorities, decided to abandon the demonstration. An estimated 3,000 people took part.
Demonstrators waving banners, joined by about a dozen tractors, had gathered earlier to protest against the WTO's three-day ministerial conference that starts on Monday.
Activists say trade policies adopted by WTO members create poverty in rich and poor countries by squeezing farmers in developing countries and depressing labour standards in industrialised nations.
Clashes have occurred at previous meetings of WTO trade heads. The last was held in Hong Kong four years ago.
swissinfo.ch with agencies