While the number of protesters was not given, reports of demonstrators being bused in from around the country caused police to deploy more than 1,300 officers, thinkSpain said.
Some 35 people were arrested in clashes with police during the "Occupy Congress" protests Tuesday afternoon.
Police used rubber bullets and batons to break up crowds attempting to tear down barricades in front of government buildings.
While police reported no widespread fighting, 64 people were injured, 28 of whom required hospital treatment. Some 27 of the injured were police officers.
The last of the protesters dispersed around midnight, but social media networks said another protest was planned for Wednesday night.