“We think that the Greek people will be courageous, that they will be on the 'No' side,” said Rommy Arce, a Madrid city councillor who belongs to a leftist “Ahora Madrid” citizens platform that took power in the capital last month.
Voting “No” to the latest bailout terms offered to Greece by the IMF and the European Union “is a rejection of all the governments that apply these policies on the rest of Europe,” she added.
“Ahora Madrid” is made up partly of Spain's anti-austerity “Indignado” protest movement that staged occupations in city squares across the country.
It was backed during local elections in May by Spain's new radical, anti-austerity party Podemos, a close ally of Greece's ruling Syriza party.
“The fundamental dilemma is between democracy and austericide,” said Jaime Pastor, a member of Podemos.
“Our future will depend on what will happen in Greece,” he added.
Podemos has not called openly for Greeks to vote “No” but one of its representatives will be in Athens on Sunday and the party has defended the right of Greeks to decide their own future.
The Greek referendum is being closely watched in Spain where anger over austerity has fuelled the rise of Podemos ahead of a year-end general election.
The party came in third place during local and regional elections in May, behind the ruling conservative Popular Party and the Socialists.
Spain's conservative government argues a replay of Syriza's policies could derail the Spanish economy's recovery from the worst downturn in its democratic history.
Spain grew by 1.4 percent last year, its first full year of growth since a property bubble burst in 2008, throwing millions of people out of work.
But its jobless rate remains at 23.78 percent in the first quarter, the highest rate in the eurozone after Greece's which stood at 25.6 percent in March.
“I don't have the slightest doubt that if Spain backtracks on reforms… we could see a very fast drop in economic growth,” Economy Minister Luis de Guindos told a news conference following a weekly cabinet meeting.
If the “Yes” side wins Sunday's referendum in Greece “it will be much more simple, easy and viable,” he added.
The minister also called Greece's decision to call the referendum “nonsense” and “a mistake” and said its effect on European leaders had been like a “cold shower”.
Prime Minister Mariano Rajoy on Tuesday said that a “Yes” vote in the referendum would be good for Greece as it would allow its creditors to negotiate with a new government in Athens.
Rallies in support of a “No” vote in the referendum are planned for Madrid and Barcelona over the weekend.
Greek Prime Minister Alexis Tsipras has said a “No” result in Sunday’s referendum would strengthen his hand and force international creditors withholding bailout funds to drop “humiliating” austerity terms.
But European Commission chief Jean-Claude Juncker warned that Greece’s negotiating position with creditors would be “dramatically weakened” in the event of a “No” vote.
Even if the “Yes” vote wins, there would still be “difficult” negotiations ahead, he added.