Rather than ordinary courts, suspected gangsters are brought before judges whose identities are hidden, ostensibly to protect them.
But sitting judge Juan Antonio Duran told AFP these were measures of "criminal populism."
He pointed out that trial by an anonymous judge, without witnesses or even the defendant present -- as has happened -- "is prohibited by the constitution."
On Monday, Amnesty International said Bukele's state of emergency "has created a perfect storm of human rights violations."
And US Secretary of State Antony Blinken reminded Bukele last week that "we can tackle violence and crime while also protecting civil rights and fundamental freedoms."
Veronica Aguirre, 26, claimed her husband was arrested groundlessly, telling AFP that under the state of emergency, "we cannot provide proof" of innocence.
Attorney General Rodolfo Delgado has insisted "honest people have nothing to fear."
But Jose Maria Tojeira, former director of the Central American University's Human Rights Institute, said El Salvador had "a strong tendency for generalized punishments which... are a source of violations of the law."
Bukele, 40, has likened El Salvador's gangs to "a metastasized cancer" and vowed there are only two paths for members: "prison or death."
For Jose Miguel Cruz, a researcher at the Florida International University, the only long-term solution was disarming and rehabilitating former gangsters and productively reintegrating them into society.
What El Salvador needed, he said, was a plan to "modify the conditions that make a good sector of the population resort to a life of crime to survive."
