Spain's riot police injured almost 900 people during referendum day, according to the official figures of the Catalan Health Ministry. The referendum, considered illegal by the Spanish authorities, went ahead anyway as thousands of people went to polling stations to try to cast their vote. Riot police raid several of those polling stations, using force excessively, as documented by HRW.

"The police may well have had the law on their side to enforce a court order but it didn't give them the right to use violence against peaceful protesters," said Kartik Raj, Western Europe Researcher at Human Rights Watch .

Spanish police engaged in "excessive force" when confronting "peaceful demonstrators" in Catalonia during the October 1 referendum, Human Rights Watch said. The NGOs carried out an on-site investigation and documented excessive use of force by Spain's Guardia Civil and national police in several locations.

"The police may well have had the law on their side to enforce a court order but it didn't give them the right to use violence against peaceful protesters

The NGO said that Spain "has an obligation to ensure an effective investigation" into the allegations, but "given the current tensions" between Madrid and Barcelona, it recommends the "involvement of an independent international expert body such as the Council of Europe, Commissioner for Human Rights or the Office of the UN's High Commissioner for Human Rights."

Human Rights Watch said that Spain "is a party to the European Convention on Human Rights as well as the International Covenant on Civil and Political Rights, which impose specific obligations with the respect to the right of peaceful assembly, freedom of expression, and use of force by law enforcement."

The NGO added that "Spain must ensure that people enjoy the right to peaceful assembly and can express their opinions freely" and "cannot deprive them of their rights" even if they "may be acting unlawfully." In fact, Human Rights Watch said that an amendment to the Penal Code in 2005 "decriminalized participation in a referendum deemed illegal by national courts."

"Human rights standards on the use of force require police to use alternatives to force wherever possible," said HRW. "The institutions of the EU and EU member states should also publicly make clear to the Spanish authorities that any force used by police has to comply with national and European human rights laws," the NGO added.