Spanish riot police smashed their way into a polling station in Catalonia on Sunday as they sought to shut down a banned independence referendum and there were reports of officers firing rubber bullets in the regional capital Barcelona.

Catalan emergency services said 38 people were hurt, mostly with minor injuries, as a result of police action.

Police burst into the polling station in a town in Girona province minutes before Catalan leader Carles Puigdemont was due to vote there. They smashed glass panels to force open the door as voters, fists in the air, sang the Catalan anthem.

Police also fired rubber bullets in central Barcelona, El Periodico newspaper reported, at the intersection of two streets as violence erupted during the vote which has thrown Spain into its worst constitutional crisis for decades.

Officers with riot shields jostled with hundreds of voters outside one station at a school in Barcelona as the crowd chanted "We are people of peace!" Armoured vans and an ambulance were parked nearby.

The referendum has been declared illegal by Spain's central government in Madrid, which says the constitution states the country is indivisible and has drafted in thousands of police from around Spain into Catalonia to prevent the vote.

The Catalan regional government had scheduled voting to open at 9 a.m. (0700 GMT) at around 2,300 stations, but Madrid said on Saturday it had shut more than half of them.

Voting started at some sites in the region of 7.5 million people, which has its own language and culture and is an industrial hub with an economy larger than that of Portugal. Leader Puigdemont changed plans and voted at a different station after the police action, the regional government said.

People had occupied some stations with the aim of preventing police from locking them down. Organisers smuggled in ballot boxes before dawn and urged voters to use passive resistance against police.

In a school used as a polling station in Barcelona, police in riot gear carried out ballot boxes while would-be voters chanted "out with the occupying forces!" and "we will vote!".

The Catalan government said voters could print out ballot papers at home and lodge them at any polling station not closed down by police.

"I have got up early because my country needs me," said Eulalia Espinal, 65, a pensioner who started queuing with around 100 others outside one polling station, a Barcelona school, in rain at about 5 a.m.

"We don't know what's going to happen but we have to be here," she said.

'I'VE WAITED 80 YEARS'

A minority of around 40 percent of Catalans support independence, polls show, although a majority want to hold a referendum on the issue. A "yes" result is likely in the referendum, given most of those who support independence are expected to cast ballots while most of those against it are not.

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.

Organisers had asked voters to turn out before dawn, hoping for large crowds to be the world's first image of voting day.

"This is a great opportunity. I've waited 80 years for this," said 92-year-old Ramon Jordana, a former taxi driver waiting to vote in Sant Pere de Torello, a town in the foothills of the Pyrenees and a pro-independence bastion.

He had wrapped his wrists in Catalan flags, among 100-150 people who gathered at a local school that had been listed as a polling station, ready to block any police from entering. A tractor also stood guard, though no police had yet arrived.

Leading up to the referendum, Spanish police arrested Catalan officials, seized campaigning leaflets and occupied the Catalan government's communications hub.

But Catalan leaders urged voters to turn out in a peaceful expression of democracy. Families have occupied scores of schools earmarked as voting centres, sleeping overnight in an attempt to prevent police from sealing them off.

"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, had 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.

But national police, who have been drafted into Catalonia in their thousands, stepped in to grab ballot boxes and close stations on Sunday once it became clear the regional police was not clearing sites.

Pro-independence 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.