Marches are being held across Spain in support of a high-profile judge accused of overreaching his powers by launching an inquiry into the Franco regime, BBC reported.
In 2008, Baltasar Garzon opened the probe - later shelved - into atrocities committed during the four-decade rule of General Francisco Franco.
A magistrate has ruled that the judge had acted without jurisdiction, and he now faces the prospect of a trial.
Protests took place in a dozen cities and thousands gathered in Madrid.
There was also a small anti-Garzon protest in the capital by members of the fascist Falange Espanola, which supported the Franco regime.
The case against Mr Garzon followed complaints by several Spanish right-wing groups.
They claimed he had knowingly exceeded his official remit in launching an investigation into tens of thousands of disappearances during Spain's 1936-1939 Civil War and under the Franco regime that followed.
'Artificial arguments'
Mr Garzon had ordered the immediate exhumation of civil war-era mass graves.
In February, Supreme Court investigating magistrate Luciano Varela ruled that Mr Garzon had ignored a 1977 amnesty that covers crimes committed during the civil war.
The amnesty law pardoned politically motivated crimes committed by both sides. By guaranteeing that the past would not be raked over, it underpinned Spain's delicate transition from dictatorship to democracy, correspondents say.
Mr Garzon appealed against the ruling.
But earlier this month, Mr Varela asserted that Mr Garzon had been aware of his lack of jurisdiction due to the amnesty law.
"Conscious of his lack of jurisdiction... he constructed artificial arguments to justify his control of the penal proceedings," he said in a written ruling.
Mr Garzon, who is highly popular among the Spanish political left and international human rights campaigners, has strongly denied that he broke the law.
But some on the right accuse Mr Garzon of launching cases that are politically motivated.
Mr Garzon is famous for targeting international figures including Augusto Pinochet and Osama Bin Laden.