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.
Related reading
Women's march keeps pressure on Venezuela's Maduro
The 50 people being held are in Guarico state, according to legal expert Luis Betancourt with Foro Penal.
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 protesters.
Clashes at fresh Venezuela anti-government demos
Riot cops fired tear gas at protesters in Venezuela Monday as thousands demonstrated against President Nicolas Maduro's efforts to reform the constitution in a deadly political crisis.
Masked protesters responded by hurling rocks and other projectiles in clashes in the capital Caracas, with other violence reported in the western states of Merida and Zulia.
Supporters of the opposition Democratic Unity Roundtable (MUD) were trying to march to the education ministry under the slogan "No to the dictatorship".
They blame Maduro for an economic crisis that has caused food shortages and say his constitutional move is a ploy to resist calls for early elections.
Education Minister Elias Jaua has been appointed by Maduro to set up an elected constituent assembly to rewrite the constitution.
Clashes at protests have left 36 people dead and hundreds injured since the unrest erupted on April 1, according to authorities.
A woman shouts slogans during a protest in Caracas, Venezuela. (AAP) Source: AAP
Jaua invited the opposition to a meeting on Monday about the constitutional reforms.
"The doors are open, especially to the spokespeople of the MUD. Dialogue is the way forward," he said.
The opposition has refused to take part in the assembly.
Opposition congressional speaker Julio Borges rejected the plan as "constitutional fraud".
Government supporters staged counter-demonstrations.
Maduro says the crisis is a US-backed capitalist conspiracy.
Private polls indicate Maduro has low approval ratings, with citizens increasingly frustrated about shortages.
"I lost my husband because we could not get hold of medicine. There is no medicine and no food," said Isabel Morales, 68, demonstrating against Maduro in Caracas.
"We cannot go on like this."
