The demonstrators, who include peasants from indigenous Andean communities, have said that the reforms would allow mining and electric firms to divert water that they use.
The protesters said that they want a special counsel to govern rights to water.
Those pushing for the reforms have said that they will regulate private water usage and ensure that the public retains access to the resource.
A second protest was held in Azuay, the Andean province. At least five leaders were arrested during the rally.