Separatists, parents and children are occupying schools set to be used as polling stations in an "illegal" referendum on Catalonia's independence from Spain.

Police have orders to close the polling stations by 6am on Sunday and have warned those occupying them that they must leave by dawn.

Huge rallies and protests have been taking place as the vote, which the national government has declared illegal, drew closer.

Scuffles between police and would-be voters broke out on Saturday and pro-Madrid nationalists rallied outside the offices of the pro-independence Catalonian government.

Spain's Constitutional Court suspended the independence vote more than three weeks ago but Catalonia's regional government is pressing ahead with it and urging registered voters to take part.

Police have been ordered to stop ballots from being cast on Sunday and have been cracking down for days, confiscating ballots and posters.

Spain's foreign minister said that the Catalan government's plan is anti-democratic and runs "counter to the goals and ideals the European Union" is trying to advance.

Prime Minister Mariano Rajoy's conservative government has said any vote on Catalan secession would have to be held across all of Spain, not just in Catalonia. The EU said Catalonia would be forced out of the bloc and have to reapply to join if it leaves Spain.

Parents supporting the referendum are camping out over the weekend at schools that are among the 2,315 designated voting facilities to stop police shutting them down.

A top Spanish security official in Catalonia said officers have already sealed off more than half of polling stations and disabled software that was to have been used in the referendum.

In Madrid, thousands of people rallied in a central plaza on Saturday to protest over the Catalan independence vote, angry that it could divide Spain.