"Put them (the politicians) in a row, or side by side, and give me an AK-47 (machine gun) and you'll see how I get rid of these fascists," the young man allegedly tweeted.
"Damn that's how you do it. Let's kill all of them," the man from Spain's Valencia region wrote in another tweet.
"Now we just need to kill (Spanish Prime Minister Mariano) Rajoy and (Deputy Prime Minister) Soraya Sáenz de Santamaría," he tweeted, before going on to list other Spanish politicians.
These tweets, posted over several days, have now landed the 19-year-old in custody.
His arrest comes after Spain's Interior Minister Jorge Fernández Díaz on Tuesday annouced authorities would crack down on people posting offensive content about murdered ruling Party Politican Isabel Carrasco.
Carrasco, the head of León's provincial government, was killed on the street near her home on Monday. A mother and daughter person were arrested shortly afterwards. On Wednesday, the mother confessed.
"We have to clean the web of these undesireable elements," said Fernández Díaz of the internet crackdown.
While some have praised the cracksown, others have viewed it as an attempt to silence free speech about a political figure widely viewed as controversial, and a futher attempt on the part of the government against criticism.
Spanish police in late April arrested 17 people, including two minors, accused of promoting terrorism and humiliating terror victims on social media sites.