The United Nations says human rights experts should be allowed to visit Catalonia in Spain.
Catalan officials claim 844 people were injured after police used rubber bullets and batons to stop a ballot for independence on Sunday.
The banned vote took place throughout the region, with violent clashes between police and voters.
During the clashes riot police stormed a polling station near Girona, to the north of Barcelona, using a hammer to smash through a glass door of a school.
The Catalan government claimed the use of batons amounted to "unjustifiable violence" which created a "terrible image of Spain".
Catalonia's government said that of the 2.2 million voters who were able to cast ballots, two million voted for independence.
Large numbers of people are expected to observe a general strike on Tuesday to condemn police violence.
UN High Commissioner for Human Rights Zeid Ra'ad Al Hussein said: "I am very disturbed by the violence in Catalonia on Sunday.
"With hundreds of people reported injured, I urge the Spanish authorities to ensure thorough, independent and impartial investigations into all acts of violence.
"Police responses must at all times be proportionate and necessary.
"I firmly believe that the current situation should be resolved through political dialogue, with full respect for democratic freedoms.
"I call on the government of Spain to accept without delay the requests by relevant UN human rights experts to visit."
On Monday, Taoiseach Leo Varadkar said Ireland will not recognise the result.
"We accept and respect the laws of Spain, the constitution of Spain and the territorial unity of Spain and it is a referendum that it would appear less than half of the population participated in.
"Although admittedly, it wasn't easy for people to participate - but that's a separate issue.
"We respect of course the laws and constitution of Spain which is a friend and ally of ours".