Venezuela’s opposition said on Monday it will hold a symbolic referendum to give voters the opportunity to reject President Nicolas Maduro’s plans to rewrite the constitution.

The plan announced by National Assembly president Julio Borges is a dramatic escalation of the opposition’s effort to fight Mr. Maduro’s proposal. Mr. Borges said that the vote would be held July 16 two weeks before Mr. Maduro is asking Venezuelans to go to the polls to choose delegates for a special assembly to overhaul the charter.

“We want the people to decide,” Mr. Borges told a crowd of opposition leaders gathered in eastern Caracas. “Today we’re united in a single bloc to defend our constitution.”

Protests against Mr. Maduro have swept across Venezuela the past three months, leaving at least 80 people dead and hundreds more jailed or injured, and the opposition-controlled legislature has embraced an agenda of civil disobedience.

Polls show that barely 20% of Venezuelans favour rewriting the late Hugo Chavez’s 1999 constitution about the same level of support for Mr. Maduro. The opposition coalition has decided to boycott the polling, arguing that rules to choose delegates to the proposed assembly are undemocratic and heavily favor the government.

Once seated, the constitutional assembly will have vast powers to reshape Venezuela’s institutions, and some in the opposition fear it could convert Venezuela into a Cuba-styled socialist system in which open elections would cease to exist.

But even with the threat of low turnout hanging over the process, the government seems determined to plow ahead.

“Even if only one, two or three Venezuelans vote, it’s still an election and the opposition will be left there squealing,” socialist party leader Diosdado Cabello said last week on state TV.

Mr. Maduro has vowed to present any new constitution for a ratification vote, but the opposition argues that a referendum is required just to call a constitutional assembly. Both times Chavez attempted to rewrite the constitution, in 1999 and 2007, he sought a popular mandate before embarking on the process.

Chief Prosecutor Luisa Ortega Diaz, an ideological admirer of Chavez who broke with Mr. Maduro, has also criticised the government’s plans. Opposition lawmakers greeted her with enthusiastic applause on Monday when she appeared at the chamber for special assembly to reappoint her deputy, who was removed last week by the government-stacked supreme court in apparent violation of the constitution

“My role as chief prosecutor requires that I assume the first line of defense of our republican state,” she said in a video address to the nation on Monday.

She said that “in the face of the attacks and threats hanging over the country,’ her agency is “perhaps one of the few democratic windows that are left open in the country” and said it “won’t be intimidated or give up the rights and freedoms of Venezuelans without a fight.”

As Ms. Ortega has assumed the role as the government’s most-feared opponent, attempts to undermine her authority have become more frequent.

On Monday, the supreme court threw out her order from a few days earlier for the former head of the national guard, Gen. Antonio Benavides, to testify about allegations of human rights abuses during the crackdown on the protests.

A few hours later, MS. Ortega responded with some strong arming of her own, saying on Twitter that her office had requested access from U.S. authorities to the case file of two relatives of first lady Cilia Flores who were convicted last year of drug trafficking.

Ms. Ortega is set to appear Tuesday before the supreme court as it debates whether to strip her of her immunity from prosecution for unspecified irregularities she allegedly committed in her role as the nation’s top law enforcement official.