Masked protesters have clashed with police at a global conference on water management in Mexico City.
Radical youths were blamed for outbreaks of violence
Police detained about 17 people as some rallies turned violent on the second day of the World Water Forum.
More than 120 countries are represented at the conference, which has pledged to focus on ways to improve access to water for the world's poor.
Mexican President Vicente Fox said water needed to be seen as a global heritage to which everyone had a right.
But protesters claim the forum is being held in the interest of big corporations and their profits, rather than that of the poor.
Most of the demonstrations in Mexico City remained peaceful, however, with the violence blamed on a small number of radical youths.
State news agency Notimex said police had arrested 17 people found carrying sticks, rocks and home-made petrol bombs, many of them wearing masks.
At least one police car was destroyed in attacks by violent protesters, the Associated Press news agency reports.
Large numbers of peaceful protesters included indigenous groups whose water is being diverted to supply big cities and those forced to live with sewage pollution.
Water is scarce in Mexico City, with many of its inhabitants managing on just one hour of running water per week.
Delfino Garcia Velazquez, a construction worker from the outskirts of the capital, said his community's scarce water resources had been taken over to supply new developments.
Most of the protests were quiet, with indigenous women leading some
"We just want to have a say over our own water and manage it ourselves, like we always have," he told AP.
Delegates at the summit heard a call for large donations to help rebuild water systems in poor countries.
Proposals for an international peacekeeping force to intervene in future conflicts over water were also put forward.
But protesters said they felt the discussion of community-level water projects that was supposed to be at the heart of the summit was being overshadowed by big companies' interests in privatisation.