The objection would then be handed to the Constitutional Court, suspending the vote until a final ruling on its legality, which could take years.
Spain's central government says a Catalan independence referendum would violate the country's 1978 constitution, drawn up on Spain's transition to democracy.
Political analysts expect the Catalan leader to call early elections after Madrid blocks the vote. He would then use the elections as a way to give Catalans a chance to vote on independence from Spain.
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Mas is under pressure from separatist coalition partners to go ahead with a referendum even if it is declared illegal, although he has himself said he would not do anything that is against the law.
Madrid's refusal to allow a vote has angered many Catalans, even those who favour continued union with Spain. Hundreds of thousands of people marched in the streets of Barcelona earlier this month for the right to hold a referendum.