BARCELONA (REUTERS) - Spanish Civil Guard police in riot gear burst into a polling station minutes before Catalan regional leader Carles Puigdemont was due to vote in a referendum on Catalan independence from Spain that has been declared illegal by Madrid.

Police used axes to smash a window at the sports centre being used as a voting centre and forced open the door in the province of Girona.

Live TV pictures showed police with black berets and riot shields fanning out through the voting station, apparently searching for ballot boxes, while would-be voters, fists raised in the air, defiantly sang the Catalan anthem.

Puigdemont had announced he would vote at the polling station near Girona at 9.30am, around the same time as the police entered, but there was no sign of him in the television footage.

He later criticised the violence used in the crackdown.

The batons, rubber bullets and violence used by Spanish police to prevent voting in what Spanish authorities have said was an illegal referendum had shown a "dreadful external image of Spain", he told reporters. "The unjustified, disproportionate and irresponsible violence of the Spanish state today has not only failed to stop Catalans' desire to vote ... but has helped to clarify all the doubts we had to resolve today," he said.

Meanwhile, Spanish national police began to seize ballot boxes and voting papers from Catalan polling stations, the Interior Ministry said, as some voters began casting ballots in Catalonia's disputed independence referendum on Sunday (Oct 1).

Riot police clashed with voters outside a Barcelona voting station, where dozens of police used riot shields to push people back, a Reuters witness said. Would-be voters chanted "we are people of peace" and "we are not afraid".

Half a dozen armoured police vans and an ambulance stood ready nearby, the witness said.

Elsewhere in the north-eastern region, some voting stations opened at 9am (3pm Sunday Singapore time) and people began to cast ballots.