The Central Electoral Commission (JEC) said demonstrations planned for Saturday and for Sunday, the day when regional and municipal polls are scheduled, are "contrary to electoral legislation."
The JEC is to inform the regional electoral commissions as well as the state attorney general of its decision.
Spanish media said the commission voted by five votes to four with one abstention in favour of the ban after six and a half hours of deliberations.
Interior Minister Alfredo Perez Rubalcaba earlier said the government was awaiting a decision from the JEC to decide whether to order police to disperse the protesters.
But he said the police are "looking to resolve problems not create more."
Young people are camped in main squares across Spain in the largest spontaneous protests since the country plunged into recession after the collapse of a property bubble in 2008.
At the spearhead in Madrid, organisers rallied thousands in the central Puerta del Sol square to decry politicians who left Spain with a 21-percent jobless rate.
Calling for "Real Democracy Now," the protests popularly known as M-15 began May 15, lamenting Spain's economic crisis, politicians in general, and corruption.
They have vowed to press on with the protests through Saturday, 'a day of reflection' ahead of the elections, and on voting day itself on Sunday.
"We are not politicians and we are not engaging in political campaigning," a spokesman for the organisers, Juan Cobo, said earlier on Thursday. And on Sunday they would "respect the electoral process."