Anti-austerity supporters crowded into Madrid's Colon Square on Saturday waving banners and demanding reform as part of the "Marches of Dignity" demonstration.
Protesters from across Spain called for an end to European Union-imposed austerity which has deepened poverty among the worst-off. Reuters said the marches brought tens of thousands to the capital in support of more than 160 different causes, including employment, housing, health, education and an end to poverty.
But as night fell, violence, injuries and arrest broke out.
Some of the protesters clashed with police following an attempt to break through a barrier to march toward the headquarters of the governing conservative Popular Party, according to The Associated Press.