Leaders in Catalonia say they are determined to go ahead with a vote on independence that Madrid calls illegal and which has thrust Spain into its most dramatic political crisis for decades.

The central government in Madrid, which has sent thousands of police reinforcements to the northeastern region to prevent people from voting on Sunday, meanwhile insisted on Friday the referendum would not happen.

Raul Romeva, Catalonia's foreign affairs chief, maintained that it is "impossible to stop" the poll, which he insisted is not illegal.

"It should concern everyone ... why the repressive response by the [Spanish] state is going this far," Romeva told Al Jazeera from Barcelona, Catalonia's capital.

He called on the Spanish government to allow Catalan voters to "solve the situation by democracy".

"If you have followed all the demonstrations taking place in Catalonia ... you'll see that it's in the DNA of the Catalan way to behave that [we] are always peaceful," Romeva said.

Courts have ordered police to cordon off schools scheduled to be used as ballot stations.

In a bid to keep them open, parents have called for a mass school sleepover this weekend, with tents and sleeping bags, free paella and cinema. Organisers said 60,000 had already registered to participate.

READ MORE: Catalan leader accuses Spain of 'totalitarian' actions

Al Jazeera's John Hendren, reporting from Barcelona, said both sides have been preparing for confrontation over the looming vote.

"Clearly, many of the polling stations will be shut down [by Spanish authorities]," he said. "The government of Catalonia has said ... it will declare itself an independent republic if they get the votes."

'Pacifist independence movement'

Separatists have called on people to turn out at the polling stations in a mass statement of "peaceful resistance", even if they are prevented from voting.

"I don't believe there will be anyone who will use violence or who will want to provoke violence that will tarnish the irreproachable image of the Catalan independence movement as pacifist," said Catalan President Carles Puigdemont.

Madrid, which claims the authority of a constitution that declares the country to be indivisible, remained staunchly opposed to the vote.

"I insist that there will be no referendum on October 1," Mendez de Vigo, central government spokesman, told a news conference following the weekly cabinet meeting, reiterating that the vote was illegal.

READ MORE: All you need to know about Catalonia referendum

Hundreds of thousands of Catalans have taken to the streets in recent weeks to protest Madrid's campaign to suppress the vote. Police have confiscated thousands of voting slips and courts have fined and threatened to arrest regional officials.

In a sign that large crowds are again expected on the streets on Sunday, department store chain El Corte Ingles said it would shut three stores in central Barcelona. The central government said airspace above the city would be partly restricted.

Lines of tractors draped in the red-and-yellow striped Catalan flag left provincial towns on Friday, planning to converge on Barcelona in a sign of support for the referendum.

Puigdemont called on the police not to act in a "political" way when carrying out their duties on Sunday.

"I would like them to use the same standards that the Catalan regional police use. Not political standards, not on political orders, but policing and professional standards," he said.