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.
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."
The government called on its supporters to stage counter-demonstrations.
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