The protest came in reaction to the dramatic eviction on Wednesday of an 87-year-old woman from the Madrid apartment she had lived in for decades.
"We're here because housing is a problem that already affects all of society, all ages and classes," said protester Dora Garcia, a 55-year-old nurse.
"And we can't keep maintaining this. Housing is a right and we don't have it."
Taken out of her home on a stretcher in the upscale Retiro neighbourhood, MarÃa del Carmen Abascal was greeted by protesters and supporters as she was loaded into an ambulance.
Spanish media reported that Abascal's father first rented the property in 1956 and she continued as the tenant after her parents died, during which time there was a rent cap.
The building changed ownership in 2018, and Abascal was offered the opportunity to buy the apartment but could not afford it, according to the Madrid Tenants' Union, which organised Saturday's protest.
Urbagestion, an investment fund that later bought the unit, raised her monthly rent from 500 euros ($A810) to 2650 euros ($A4293). It subsequently lowered the rent to 1650 euros, which Abascal still could not afford on her pension, the union said.
Courts blocked three previous attempts to evict her. Extensive media coverage turned Abascal into a symbol of Spain's housing crisis.
"This whole housing situation is heartbreaking. Maricarmen has been, in a way, the last straw that sparked this social unrest, but we've been dealing with this whole issue for a long time now," said protester Simon de la Riva, 43, a teacher.
"It's unacceptable that vulture funds are buying up homes, that people can't afford to live, that entire families are being evicted, and that an elderly woman has been evicted."
At the end of the march, protesters sat down in the Puerta del Sol plaza with plans to stay put. Some even pitched tents to stay at least the night - or until police forcibly removed them.
Rising costs and a housing shortage are pricing many Spaniards out of the market, despite strong economic growth in Europe's fourth-largest economy. Incomes have not kept pace.
Analysts say tourism and immigration-driven population growth in cities have placed further pressure on the limited housing supply.
The Bank of Spain estimated the country had a shortage of about 550,000 homes last year, with Madrid, Barcelona and Valencia accounting for a substantial share.
Spain ranks near the bottom of countries in the Organisation for Economic Co-operation and Development for public rental housing, which represents less than two per cent of its available supply. The European Union average is eight per cent.
