Protesters, many waving Spanish flags and holding signs that read “Sanchez traitor” and “Don’t sell Spain”, demonstrated against the law which four judicial associations, opposition political parties and business leaders said threatens the rule of law and the separation of powers.
Alberto Nunez Feijoo, leader of the opposition conservative People’s Party, and Santiago Abascal, leader of the far-right Vox party, also attended the march which was organised by civil groups.
After the rally, scores of protesters walked along the side of a motorway towards the Moncloa Palace, the prime minister’s residence in Madrid.
The amnesty will cover about 400 people involved in the independence bid that came to a head in 2017, including separatists but also police involved in clashes with activists.
The independence referendum was declared illegal by the courts and resulted in Spain’s worst political crisis for decades.
The amnesty will be the largest in Spain since the 1977 blanket amnesty for crimes committed during the Francisco Franco dictatorship, and the first amnesty law approved in the European Union since 1991, according to Spain’s CSIC research council.
Sanchez, who won a parliamentary vote to form a new government on Thursday by 179 votes in favour and 171 against, has defended the law saying an amnesty would help to defuse tensions in Catalonia.
Protesters, including neo-Nazi groups, have held rowdy demonstrations outside the Socialist headquarters in Madrid for 15 nights consecutively since the deal was announced. There have been clashes with police which left officers and demonstrators injured but in general, the protests have been peaceful.
In a survey by Metroscopia in mid-September, around 70% of respondents – 59% of them Socialist supporters – said they were against the idea of an amnesty.