BARCELONA (Reuters) - Catalan separatists urged supporters to defy Spanish efforts to block an independence referendum on Sunday, calling for peaceful turnouts at polling stations that police have been ordered to keep shut. Pro-independence groups the Catalan National Assembly (ANC) and Omnium said on Thursday that people should form queues if they found police guarding voting stations, amid concerns that frustrations over an event progressively stripped of any meaningful political impact could erupt into street unrest. 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 is pressing ahead, and its leader Carles Puigdemont - who has labeled 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.  With both groups having strong track records of non-violent protest, the biggest risk of civil disturbance appeared to lie with members of foreign anarchist groups, who local newspapers including El Confidencial and El Espanol said had arrived in Barcelona. If they approach any of the more than 2,500 voting stations across Catalonia, they will encounter a stepped-up police.  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.  We are concerned that this order and the accompanying rhetoric may heighten tensions and social unrest,  said David Kaye, Special Rapporteur on the promotion and protection of the right to freedom of opinion and expression, and Alfred de Zayas, independent expert on the promotion of a democratic and equitable international order.     Following a meeting a 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. ANC said voters should show  institutional dignity  and form queues without staging a  spectacle . 