A top United Nations official on Monday called for an independent and impartial investigation into the violence that saw hundreds injured as Spanish authorities grappled to thwart a Catalan independence referendum ruled illegal by Spain's judiciary.

UN High Commissioner for Human Rights Zeid Ra'ad Al Hussein voiced his concern over the situation in Catalonia after more than 800 people were injured, according to Catalan authorities, during clashes with Spanish police on Sunday.

"I am very disturbed by the violence in Catalonia on Sunday," the official said. "With hundreds of people injured, I urge the Spanish authorities to ensure thorough, independent and impartial investigations into all acts of violence."

He said, "police responses must at all times be proportionate and necessary."

Members of Spain's National Police and Civil Guard on Sunday clashed with voters during efforts to confiscate ballot boxes and prevent the referendum from taking place.

Spain's Constitutional Court had ruled the vote was illegal.

Prime Minister Mariano Rajoy declared that no referendum had taken place in Catalonia on Sunday, while the Catalan government said that one had not only taken place, but the case for independence had prevailed, paving the way toward a unilateral declaration of independence.

"I firmly believe that the current situation should be resolved through political dialogue, with full respect for democratic freedoms," said Zeid.

He also called on the Spanish government to "accept without delay" requests by UN human rights experts to visit.