The 38-year-old, who claimed health professionals and the media were behind what he called the “Covid farce”, urged his social media followers to attack politicians and journalists, police said.
“All this would be solved with a shot to the back of (Spanish Prime Minister) Pedro Sanchez’s head,” he wrote via one of several anonymous accounts.
In other posts he described virus “believers” as bad and ignorant people who deserve to die.
Passing himself off as a government official, police said the suspect allegedly made calls to nursing homes, hospitals and football clubs to spread false information about the pandemic.
Detectives were able to trace the man, described as a “grave danger” to public health, to a location just outside Zaragoza, the capital of Aragon.
It has suffered the highest infection rates in the country over the past two weeks, with 404 cases per 100,000 people.
On Thursday, Spain reported 3,781 new cases, bringing the cumulative total to 429,507 – the highest in western Europe.
During the epidemic’s peak, Spain imposed one of Europe’s strictest lockdowns and introduced strict face covering rules when it was lifted at the end of June.
While compliance with national restrictions remains the norm, a small but vocal anti-mask movement has been gaining prominence in the past few months, spurred on by the endorsement of 1980s pop star Miguel Bose.