Amnesty International believes the verdict on the referendum leaders is "against human rights international law."

A report published on Tuesday reads that the NGO is concerned about the "criminalization" of peaceful protests, which could have a "dissuasive effect" on future citizen demonstrations.

Nine politicians and activists were convicted to between 9 and 13 years in jail for their role in the 2017 Catalan independence push.

Two of them were at the time leaders of the main civic associations in favor of a Catalan republic, Jordi Cuixart and Jordi Sànchez, and as leaked on Saturday, Amnesty demands their immediate release.

The non-governmental organization had already been asking for their release during their two-year provisional detention before they were given their sentence.

Amnesty has also expressed concerns that the verdict cited the ''mass'' demonstrations as a basis for the crime of sedition, because this would imply that ''there is a limit on the number of people who can simultaneously exercise their right to peaceful assembly.''

The human rights group also questions the crime of sedition, asking Spain's authorities to ''review'' it - seven more leaders as well as Sànchez and Cuixart have been convicted.

For a full rundown of the most important sections of the report, click here.

Catalan parties hot and cold over report

It did not take long for Catalonia's political parties to react to the Amnesty report, with the pro-independence JxCat party calling it "a slap in the face" for the state, with spokesman Eduard Pujol asking: "How many times must the Spanish government be embarrassed."

Meanwhile, MP Carles Riera from the pro-independence far-left CUP party called the report "conclusive" and said it signalled "a political victory for civil and political rights" and "a warning and defeat for the Spanish state."

Yet, unionist parties were not so enthusiastic, with Eva Granados of the Catalan Socialists, the same party as the acting government in Madrid, saying "the sentence should be respected, and the report from Amnesty International is no more than just another opinion."

The Catalan wing of the conservative PP party went further, with MP Santi Rodríguez calling the report "out of order" and accusing it of being "propaganda," while pointing out that the court's verdict on the independence leaders is final and "not a political decision."