BARCELONA, Oct 1 (Reuters) - 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.

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 northeastern region, some voting stations opened at 9 AM (0700 GMT) and people began to cast ballots. (Reporting by Sam Edwards; Editing by Adrian Croft)