Tensions are heating up ahead of the vote scheduled for October 1 despite opposition from Madrid and a Supreme Court ruling calling it illegal.
The mayors have been called in for questioning by prosecutors for agreeing to facilitate the vote locally, the report said.
The Catalan government has been scrambling to push forward the vote, despite the central government's warnings that local municipalities are not allowed to use public buildings for it and mayors can be legally prosecuted for it. Organizing the referendum will be almost impossible without the cooperation of local municipalities.
Along with hundreds of flag-waving protesters in downtown Barcelona, the mayors gave speeches in which they promised continued support for the referendum.
"We will not be intimidated".
Organisers said 35,000 people rallied in the northern city of Bilbao - a symbolic protest in a region still marked by decades of violence once waged by armed separatist group ETA, and where the desire for independence remains strong. "Eighty percent of the Catalan citizenship is in favor of the referendum of self-determination to ask Catalans whether or not they want independence", Anna Arque from the EU Partnership for Independence told RT.
She pointed to the Edinburgh Agreement, signed in the build-up to the Scottish independence referendum, as to how governments can work together in such circumstances. It is unclear what arrangement was reached.
Brandishing maces, chanting "we will vote" and singing "Els Segadors", the regional anthem, the mayors were cheered on by supporters waving yellow, red and blue Catalan flags.
On Saturday, Prime Minister Mariano Rajoy vowed to block the vote. Catalonia's top court has also warned seven newspapers against publishing campaign material.
The decision over Catalonia's future is a "matter for the people who live there", the Scottish Government has said as the row continues over plans for an independence referendum in the Spanish region. Taxes, foreign affairs, defence and infrastructures are in the hands of Spain's central authorities.