One lawyer working for those detained, Tony Marval, said 70 of them were being held in the northern state of Carabobo on the order of military courts.
The non-governmental criminal justice body Foro Penal said a further 11 were in a similar plight in Caracas and the northwestern state of Lara.
President Nicolas Maduro has yet to respond to the statement, which has raised claims of an authoritarian turn in Venezuela's political crisis.
Government officials have not confirmed the arrests, or the military processing of civilian suspects.
Opposition lawmakers passed a motion Tuesday condemning the detentions.
"The constitution is clear: military courts are not for civilians. Bringing demonstrators before them is a violation of their human rights," said the opposition speaker of the legislature, Julio Borges.
The National Assembly resolution condemned the military detentions and charging of civilian protestors, calling it a human rights violation.
"That is something that dictatorships do," Organization of American States chief Luis Almagro said from Washington.
The Uruguayan is the most outspoken international critic of Maduro. The Venezuelan president brands him a US puppet.
Deadly clashes
Clashes between protesters and riot cops 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.
His move to reform the constitution has further inflamed protesters, who say it is a ploy to resist calls for early elections.
Maduro says the crisis is a US-backed capitalist conspiracy against his elected socialist government. He has branded protesters "terrorists" and insurgents.
Hundreds detained
A senior military commander, Jesus Suarez, said that 780 people had been arrested in protests.
He said 251 of them were sent to military courts for charges such as attacking security forces and "rebellion."
Constitutional law expert Jose Vicente Haro said the detentions violate article 261 of the Venezuelan constitution which says military courts can only handle "crimes of a military nature."
"This is a violation of citizens' right to be judged by their peers in a civil manner with due process," Marval told AFP by telephone.
Robbery, 'rebellion'
Alfredo Romero, an attorney with Foro Penal, told AFP that many of the detainees were arrested for robbery but face charges such as rebellion and contempt.
Venezuela's chief prosecutor Luisa Ortega has broken ranks with the government to speak out against detentions of protestors.
Human rights groups said the military courts were a way to try cases that she had dismissed.
Maduro has the public backing of the military high command -- a decisive factor in the political crisis, analysts say.
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