The ruling Tory PP lost control of more than 12 provincial capitals and many smaller towns as 8,000 local governments were formed around the Spanish state last weekend.
Local left wing activist coalitions took office in four of the five biggest cities, among others.
Communist judge Manuela Carmena was made mayor of Madrid, alongside direct action anti-eviction campaigner Ada Colau in Barcelona, human rights lawyer Pedro Santisteve in Zaragoza and lecturer Joan Ribo in Valencia.
In a fractured political landscape it took weeks to form local governments after the elections. Most of the devolved parliaments are still without regional governments after elections produced no clear winners.
A new right wing liberal party Ciudadanos also made inroads into the mainstream in its first major electoral test outside Catalonia. Its support has been vital for the traditional main parties where they have clung to office.
The gains for the radical left could point to opportunities for the Podemos party in the general election set to take place next year.