Madrid (AFP) - Spain's Civil Guard police force on Tuesday searched the headquarters of Catalonia's regional police, the Mossos d'Esquadra, in a probe centred on the outlawed independence referendum on October 1, a spokesman said.

"We're carrying out an inspection related to the Mossos d'Esquadra's communications on the day of the illegal referendum on October 1," the Civil Guard spokesman told AFP.

He said officers were searching the headquarters in Sabadell, a city in Catalonia, and other offices.

The Mossos were tasked with stopping the ballot, which had been deemed illegal by Spain's Constitutional Court.

But senior Mossos officers issued orders not to use force, and agents steered clear of clashes with activists gathered in large numbers to protect voting stations.

Members of the national police intervened instead, seizing ballot boxes in a clampdown marred by violence against civilians.

The Mossos have since been criticised for not closing down polling stations as they had been asked to. They retorted that they shut as many as they could.