Alfredo Romero, an attorney with the NGO Foro Penal (Criminal Justice Forum), told AFP that military hearings involving civilian suspects were known to have been taking place for several days.
"So far, 75 people have been brought before Venezuelan military courts. Fifty of those remain in custody," said Romero, who is representing the detained civilians.
Another forty or so people were expected before judges later Monday, said the lawyer, who denounced what he called an "illegal" bid by a military court to try civilians.
Government officials have not confirmed the arrests, or the military processing of civilian suspects.
The 50 people being held are in Guarico state, according to legal expert Luis Betancourt with Foro Penal.
Luis Almagro, the head of the Organization of American States, lashed out at the reported detentions in a video.
"Venezuela's part military, part civilian regime represents the very worst of a dictatorship," Almagro said.
In particular, "having military judges charge civilians is utterly out of proportion, legally speaking," said Almagro, a Uruguayan diplomat who leads the US-based regional policy coordinating body.
Riot cops fired tear gas Monday as thousands pressed ahead with a weeks-long campaign of protest against Maduro's efforts to reform the constitution.
Clashes at protests have left 36 people dead and hundreds injured since the unrest erupted on April 1, according to authorities.
Demonstrators blame Maduro for an economic crisis that has caused food shortages in the oil-rich state and say his constitutional move is a ploy to resist calls for early elections.
Maduro says the crisis is a US-backed capitalist conspiracy against his elected socialist government.
His own chief prosecutor has spoken out against the detention of protestors.
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