In the council's statement, UN human rights experts say, "Regardless of the lawfulness of the referendum, the Spanish authorities have a responsibility to respect those rights that are essential to democratic societies."
The statement notes that authorities have searched printing establishments, seized referendum material, blocked websites, stopped political meetings and deployed more than 4,000 police officers to the Catalan region. They also express concern that leaders of the mass protests have been charged with sedition and about the arrest of politicians.
"The measures we are witnessing are worrying because they appear to violate fundamental individual rights, cutting off public information and the possibility of debate at a critical moment for Spain's democracy," the UN human rights experts say.
The experts, "called on the Spanish authorities to ensure that measures taken ahead of the Catalan referendum on 1 October do not interfere with the fundamental rights to freedom of expression, assembly and association, and public participation," adding that they have been in contact with the Spanish government.
The experts are David Kaye, the UN special rapporteur on freedom of expression, a former U.S. state department official, and Alfred de Zayas, a UN independent expert on democracy and a U.S. lawyer.
WILL POLLING STATIONS OPEN ON SUNDAY?
Students protested in Barcelona Thursday against the measures Madrid is taking to block the referendum, while pro-independence groups, the Catalan National Assembly (ANC) and Omnium, said that people should go to voting locations very early on Sunday and form queues if they find police guarding polling stations.
Tackling one of the biggest political crises to hit Spain since democracy was restored in the 1970s after decades of dictatorship, authorities in Madrid have declared the referendum unconstitutional and told police to ensure no votes are cast.
The rich northeastern region, which has a population of 7.5 million, is pressing ahead, and its leader Carles Puigdemont — who has labelled the government's response anti-democratic — said a week ago he had contingency plans in place to ensure the vote would take place.
But ANC and Omnium said Catalonia's priority for Sunday should be to present a responsible and united face to the world, even if that meant forming long queues without actually voting.
"Peaceful resistance, zero violence.… If you can't access the voting stations, by no means should you respond with violence," ANC said in an internal document distributed to members.
"Above all, bear in mind this is not a demonstration but a giant queue. The picture of millions of people queuing with a ballot paper in their hand will be more impressive."
Concerned about reports that police have been ordered to seal off voting sites, a new group, Open Schools, has formed to help keep schools open over the weekend by organizing concerts and other activities. On Thursday night, the group claimed to have already signed up 50,000 volunteers.
Around 4,000 state police from other regions have been deployed to prevent the vote and maintain security. They will join 5,000 state police based in the region and 17,000 local police, or Mossos d'Esquadra.
The Mossos have said the order to close voting stations increased the risk of confrontation between demonstrators and police, a worry shared on Thursday by two United Nations experts.
Following a meeting in Barcelona of senior security officials, Spain's junior interior minister Jose Antonio Nieto confirmed no vote would be allowed, though the government would not prevent people from demonstrating.
"On Sunday, it will be possible to celebrate, everybody in a different way, through a picnic or a demonstration, and to express a sentiment, but there will be no breach of the law," he told a news conference.
EU ACCUSED OF 'TURNING ITS BACK' ON CATALONIA
Also on Thursday, Catalonia's foreign affairs chief, Raul Romeva, called for the European Union to support the referendum, echoing an appeal by Catalan regional President Carles Puigdemont. Puigdemont has accused the EU of "turning its back" on Catalonia in its conflict with Spain's central government.
Barcelona Mayor Ada Colau also urged the EU to safeguard "fundamental rights and freedoms in Catalonia" by mediating the standoff between Catalan officials and national officials in Madrid before Sunday's planned vote.
"If Europe wants to have credibility as a project, it needs to redirect this situation," Colau told the Associated Press in an interview.