Officers from the Guardia Civil police force also paid a visit to several town halls to ask them to hand over documents related to the banned vote on October 1st, the Association of Pro-Independence Municipalities told AFP.
These are the latest in a series of measures taken by police and the courts to try and stop the referendum from taking place in a region sharply divided over independence.
A police spokesman said the 15 questioned were accused of allegedly “disseminating a website for people to participate in a referendum declared illegal by the Constitutional Court.”
He added they allegedly crafted web platforms that “made it easy for people to get documents… to organise the plebiscite.”
They will be released pending further questioning by a judge, he said.
Catalan President Carles Puigdemont has tweeted several links to websites directing people where to vote on Sunday, but these have been taken down after a court ordered sites that facilitate such information be blocked.
Meanwhile Miquel Sala, mayor of Oliana in northwestern Catalonia, said three Guardia Civil officers had visited on Monday morning.
Sala is one of more than 700 mayors who have pledged to cooperate with the referendum.
Absent at the time, he said his staff only handed over a report compiled for internal use summing up Madrid's position on the vote.
Sala has also signed a manifesto pledging his cooperation for the referendum but refused to hand that over and will only do so if a judge asks, he added.
The measures taken by Madrid have contributed to a spike in tensions in Catalonia.
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They have also dealt a blow to separatists leaders' plans to hold a referendum with a semblance of legitimacy.
Key members of the team organising the vote have been put under official investigation for disobedience, malfeasance and embezzlement of public funds.
Organisers of protests last week have been threatened with charges of sedition.
And police have seized close to ten million ballot papers, as well as other items destined for the vote.
This has prompted the Catalan regional government to accuse central authorities of imposing an “undeclared state of emergency.”