Protesters threw Molotov cocktails and troops fired rubber bullets in the air Friday afternoon in one part of the capital, Caracas, video distributed by Reuters showed. Dozens of people were arrested, Reuters reported.
Maduro's regime has forbidden protests through Tuesday, saying violators will face prison terms of five to 10 years. It says it has dispatched more than 370,000 troops across the country to secure Sunday's vote.
The opposition wants to stop the election, which likely would favor Maduro as his opponents largely are not running.
Opposition leaders have called for protesters to gather Sunday at a freeway in Caracas and main streets throughout the country.
"What are they going to do? Arrest millions of people who protest?" opposition leader Henrique Capriles told reporters Friday.
Protesters blocked more than a dozen intersections Friday in Caracas. In one instance, about 120 people gathered at and near an intersection in the Altamira neighborhood.
People wearing masks blocked the intersection, in part with bricks, wire, tires and other debris on the street. Some held signs critical of Maduro.
At one protest site, paramedic Johann Paredes, 27, helped both injured protesters and national guard troops Friday. He says he tries to be apolitical.
"But if I take my helmet off, I would say I'm totally against" the vote, he said. "Honestly, I'm terrified that (it) is going" forward. "It's a disaster."
The election comes after months of sometimes deadly protests as an economic crisis
has led many to flee the country in search of easier access to food
Maduro, who has said that rewriting the constitution is needed to restore order, told a rally earlier this week in Caracas that he has proposed talks with the opposition.
Opposition leaders have said they are willing to talk only if Sunday's vote is postponed. Maduro on Saturday claimed that there were discussions that brought the two sides close to agreement, but talks fell apart. He blamed opposition leader and National Assembly Vice President Freddy Guevara for the breakdown.
Sunday's election: What's at stake
Voters Sunday will elect a 545-seat assembly that could do whatever it wants in rewriting the constitution -- including abolishing the current National Assembly and even removing Maduro, said Luis Vicente Leon, a Venezuelan political analyst and college professor.
But Maduro called for the constitutional revisions, and with pro-Maduro candidates running for many of the seats, including former ministers in his government, the body is likely to favor him. The assembly could give Maduro new powers and dissolve state institutions.
Though the opposition won control of the National Assembly in 2015 elections, it is essentially void because pro-Maduro legislators stopped attending sessions.
More than 350 members of the new assembly are to be elected in open municipal votes. The remaining members will be elected by people from certain social and industry groups (like students, pensioners or workers).
Voters are being told they can go to any polling station in their municipalities in case protesters block access.
Cilia Flores, Maduro's wife and a candidate for the new assembly, has said the new pro-Maduro members will install a separate justice commission "to determine those responsible for these terrible damages," including politicians who she says has promoted violence.
In a nonbinding July 16 referendum organized by opposition parties, an overwhelming majority of voters came out against Maduro's plan.
The referendum asked voters three yes-no questions. More than 98% of voters chose to reject the proposed constitutional assembly, request the military defend the existing constitution and support fresh elections before Maduro's term ends in 2019.
The turnout represented about 37% of Venezuela's total electorate.
Colombia: New assembly won't be recognized
International pressure against Venezuela's election has been increasing. In the past few days, the United States
, Mexico and Colombia said they're freezing assets and imposing other restrictions on certain current and former Venezuelan government officials.
The United States would follow up with further "strong and swift economic action"
if the vote happens, senior US officials have said.
Colombian President Juan Manuel Santos said Friday his government will not recognize Venezuela's constituent assembly.
The assembly has a "fake origin and therefore, the results will not be recognized," Santos said in Barranquilla.
Colombia is pushing for a "peaceful solution, hopefully a fast and democratic one, so that the nation that we love -- and I want to once again express my solidarity with the people of Venezuela -- will soon come out from the darkness," Santos said.
The relationship between the neighbors has been strained for years.
Maduro on Saturday called Santos a slave of the United States.
For months, violence has spiraled out of control as the struggle for food and medicine grows.
Political upheaval intensified in late March when the Venezuelan Supreme Court dissolved Parliament and transferred all legislative powers to itself. The court reversed that decision after three days, but the initial move triggered intermittent, deadly protests for months.
As of Friday, at least 113 people have died in protests and other incidents linked to the unrest, the attorney general's office has said, often without elaborating on who was responsible for their deaths.
At least seven of those deaths happened Wednesday and Thursday during a two-day opposition-led general strike
Maduro and Chavez
Maduro is aligned with the political movement of Hugo Chavez, the President from 1999 until his death in 2013. Under constitutional changes Chavez ushered in, presidents can run for an unlimited number of six-year terms.
Chavez trumpeted a brand of socialism -- dubbed Chavismo -- in which he increased subsidies to the poor and fixed prices for goods, but alienated Venezuela from foreign investors who were spooked by his anti-American rhetoric. Maduro, elected as Chavez's successor in 2013, kept up his predecessor's practices.
Venezuela became dependent on selling its oil abroad, and income suffered
when the price of oil per barrel dropped from $100 in 2014 to $26 in 2016. Inflation has soared, and unemployment could reach 25% this year, according to the International Monetary Fund.
Maduro has long verbally clashed with opposition leaders. The opposition wanted to impeach Maduro after it won a National Assembly majority, but he stacked the Supreme Court with his supporters, blocking any impeachment attempts.