CARACAS, Venezuela  — Institutions loyal to Venezuelan President Nicolas Maduro encroached further on his opponent’s dwindling power base in the nation’s government on Tuesday, taking over the halls of the endangered, opposition-controlled congress and sentencing a mayor at the center of recent protests to prison.

Delegates to the new, all-powerful constitutional assembly convened in the stately, gold-domed chamber where congress normally meets in another sign that it intends to muscle aside any authority still held by the opposition.

Opposition lawmakers said they were barred from entering the legislative palace after security forces led by constitutional assembly president Delcy Rodriguez broke into congress late Monday to set up seats for the 545 pro-government delegates.