The UN's human rights chief is calling on the Spanish government to ensure "thorough, independent and impartial investigations" into acts of violence linked to the vote.
Zeid Ra'ad al-Hussein said he was "very disturbed" by Sunday's violence in Catalonia. He said police responses must "at all times be proportionate and necessary".
Riot police kicked a hit voters with batons, fired rubber bullets and were seen in videos to throw people down stairs and drag them by the hair. More than 800 people were injured, some seriously.
The UN's rights chief said the situation should be resolved through political dialogue.
Zeid, a Jordanian prince who goes by his first name, also urged Madrid to accept "without delay" the requests of two UN-mandated investigators on freedom of assembly and minorities to be granted access to visit Catalonia.
Rights office spokesman Rupert Colville said the two UN "special rapporteurs" had previously sought the access before the weekend's violence.
Meanwhile, Catalonia's leader has called for international mediation to solve the political deadlock.
Carles Puigdemont also called on Spain's national police reinforcements to leave the north-eastern region, a day after the violence broke out as police moved to stop voting.
He said the regional government will investigate responsibilities over rights violations.
Puigdemont called for the European Union to consider Catalonia's desire to break away from Spain as a regional problem, and urged Mr Rajoy's government to accept mediation.
The Catalan president said the regional parliament will be carrying out in the next days the mandate to declare independence in the light of the "yes" victory in the referendum.
Earlier, the European Union's executive urged all sides in Spain "to now move very swiftly from confrontation to dialogue" in the wake of the violence.
EU spokesman Margaritis Schinas also said the Commission trusts the leadership of Spanish prime minister Mariano Rajoy "to manage this difficult process".
Schinas said: "These are times for unity and stability. We call on all relevant players to now move very swiftly from confrontation to dialogue. Violence can never be an instrument in politics."
He said EU Commission president Jean-Claude Juncker and Mr Rajoy would engage in talks later.
Catalan officials said the poll, which Spain insists is illegal and invalid, showed that a majority who voted favour independence.
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