The referendum, declared illegal by Spain's central government, has thrown the country into its worst constitutional crisis in decades and raised fears of street violence as a test of will between Madrid and Barcelona plays out.
Civil Guard national police also streamed in a convoy through the streets of Barcelona in the early morning, but at dawn there was no sign of either national or Catalan police enforcing a court order to lock down the polling stations.
At some stations, voters blocked doors in anticipation that police could try to enter and take over the sites. At one, a Barcelona school, organisers asked people to use passive resistance if police intervened.
"I have got up early because my country needs me," said Eulalia Espinal, a 65-year-old pensioner who started queuing with around 100 others outside one polling station, a Barcelona school, in rain at about 5 a.m. (0300 GMT).
"We don't know what's going to happen but we have to be here," she said.
Organisers had asked voters to turn out hours before polling stations were supposed to open at 9 a.m., and called for "massive" crowds by 7.30 a.m., hoping for this to be the world's first image of voting day.
Leading up to the referendum, Spanish police arrested Catalan officials, seized campaigning leaflets, sealed off many of the 2,300 schools designated as polling stations and occupied the Catalan government's communications hub.
But Catalan leaders, backed by pro-independence supporters, have refused to abandon their plans. Families have occupied scores of schools earmarked as voting centres, sleeping overnight in an attempt to prevent police from sealing them off.
If some voting goes ahead, a "yes" result is likely, given that many unionists are not expected to turn out.
"If I can't vote, I want to turn out in the streets and say sincerely that we want to vote," said independence supporter Jose Miro, a 60-year-old schools inspector.
Only the Catalan police, or Mossos d'Esquadra, have so far been monitoring polling stations. They are held in affection by Catalans, especially after they hunted down Islamists accused of staging deadly attacks in the region in August.
Pro-independence Catalan leader Carles Puigdemont originally said that if the "yes" vote won, the Catalan government would declare independence within 48 hours, but regional leaders have since acknowledged Madrid's crackdown has undermined the vote.
Markets have reacted cautiously but calmly to the situation so far, though credit rating agency S&P said on Friday that protracted tensions in Catalonia could hurt Spain's economic outlook. The region accounts for about a fifth of the economy.
The ballot will have no legal status as it has been blocked by Spain's Constitutional Court, and Madrid has the ultimate power under its 1978 charter to suspend the regional government's authority to rule if it declares independence.
The Madrid government, which has sent thousands of police to Catalonia to enforce a court ban on the vote, believes it has done enough to prevent any meaningful referendum taking place.
Farmers have used tractors to guard polling stations in 30 Catalan towns, according to Spanish media reports. They included one at a sports centre in Sant Julia de Ramis, near Girona, where Puigdemont was scheduled to vote later.
At other polling centres, activists carried away schools' iron gates to make it harder to seal them off.
A minority of around 40 percent of Catalans support independence, polls show, although a majority want to hold a referendum on the issue. The region of 7.5 million people has an economy larger than that of Portugal.
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