https://bbc.com/news/world-europe
About 7,000 demonstrators gathered outside Mr Sánchez's Socialist party headquarters in Madrid on Tuesday.
He is preparing for an investiture vote which is expected to see him form a new government and avoid a repeat election.
That can only happen if he can secure the parliamentary support of Catalan separatists.
As police and protesters clashed on Tuesday night, Mr Sánchez posted a defiant message on social media: "They will not break the Socialist Party."
Similar protests have been taking place in the Spanish capital and in other cities on a daily basis recently, although this was the most violent so far, with 29 police and 10 demonstrators getting injured.
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