Large swaths of Caracas, the capital of Venezuela, were shuttered and silent on Thursday as opponents of President Nicolas Maduro called the first major national strike since a 2002 stoppage that failed to topple Maduro's predecessor Hugo Chavez.

A public transport strike appeared to have halted nearly all bus traffic, and thousands of private businesses defied government demands to stay open. State-run firms were open, though many were short on staff after employees failed to appear. Improvised roadblocks closed many streets.

Security forces fired tear gas at protesters manning barricades. Youths shot fireworks at them from homemade mortars.

Ronney Tejera, 24, and Andres Uzcategui, 23, died after being shot during protests, the state prosecutor's office said. More than 170 people were arrested by late afternoon, a local rights group said.

The 24-hour strike was meant as an expression of national disapproval of Maduro's plan to convene a constitutional assembly that would reshape the Venezuelan system to consolidate the ruling party's power over the few institutions that remain outside its control. The opposition is boycotting a July 30 election to select members of the assembly.

"Definitively, we need a change," teacher Katherina Alvarez told the Associated Press news agency. "The main objective is for people to see how dissatisfied people are."

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The country's largest business group, Fedecamaras, has cautiously avoided full endorsement of the strike, but its members have told employees that they won't be punished for coming to work.

Fedecamaras played a central role in the months-long 2002-2003 strike that Chavez's political rivals and opponents in Venezuela's private business sector orchestrated in an attempt to topple him.

Chavez emerged from the strike and exerted control over the private sector with years of expropriations, strict regulations and imports bought with oil money and meant to replace local production. Business groups estimate that 150,000 Venezuelan businesses have closed over the last 15 years. The opposition called a 12-hour national strike last year that saw little response and was widely seen as a failure.

"This is a work stoppage by civil society. He who wants to work, work. Who wants to stop, stop," said Francisco Martinez, the president of Fedecamaras.

Striking businesses 'will be punished'

Government-run industries will remain open, and Labour Minister Nestor Ovalles said the Maduro administration would punish private companies that close in sympathy with the strike.

"We won't allow, and we'll be closely watching, any disruption that violates the working class' right to work," Ovalles said. "Businesses that join the strike will be punished."

The business group's incoming president, economist Carlos Larrazabal, said the strike would be of limited duration to avoid worsening Venezuela's already dire shortages of food and other basic products.

"Inventory levels right are very precarious," Larrazabal said. "If the supply chains are affected more than they are right now, we could have a bigger problem."

However, the Venezuelan Workers' Confederation, a labour coalition with ties to the opposition, said at least 12 of its 20 member organisations across the country had decided to join the strike. Transport workers in Caracas also said they would participate.

"There's an appeal to the conscience of the Venezuelan people," said Pedro Jimenez, head of a major transport workers' union. "There won't be transportation services."

Many of those who opted to work said they walked hours to get to their jobs, unable to find a bus or taxi.

"We urgently need a change in government, because what we are living through is pathetic," said Frangeli Fernandez, 24, an accountant who walked three hours to his job at a bank.

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Although not entirely in agreement with the strike, Fernandez told AP news agency he agreed with doing something "very radical to get out of this".

Al Jazeera's John Holman, reporting from Cucuta on Colombia's border with Venezuela, said that there were reports that in many states 60 to 80 percent of businesses had shut.

"So in that way, it does seem to have had quite an impact nationwide - and more so than general strikes in the past in Venezuela," said Holman.

"But at the same time this is a country with large economic problems, with chronic shortages of basic products, raw materials, companies failing already - and some people have questioned if a strike is really the best way to go about protesting against this constituent assembly," he said.

'A lot of pieces on the chessboard'

Walking through the streets of Venezuela's capital Thursday, opposition leader Henrique Capriles called on people at roadblocks to allow ambulances and other emergency vehicles through. Protesters across Caracas set up roadblocks of tree branches and tires to protest against Maduro's plans to change the constitution.

Some residents were irritated by the roadblocks, saying the protest was yet another way the country's political upheaval is disrupting their lives.

"The government jails the people who protest and those who are protesting are caging the rest of us. It's unfair," said Maria Sandoval, a 27-year-old medical secretary.

But those at the roadblocks said they had no plans to stop until Maduro leaves power.

"The people don't want this government," accountant Wilfredo Villegas said as he manned a roadblock with neighbours on Thursday morning.

"We're here making them understand that we don't want this government. They have to go, in a non-violent way. They have to call general elections."

Al Jazeera's Holman said the strikes could affect ordinary people more than the government, and that the strike alone won't change things.

"[But] it's part of a concerted push by the opposition over the next two weeks - and a key part of that is support from the international community ... and also the military and police."

If rifts emerge in the police and military it could have a decisive impact, said Holman.

"So there are a lot of pieces on the chessboard right now, and the opposition are trying to use them all to put more and more pressure on this government."

Venezuela UN diplomat resigns in protest

Meanwhile, a Venezuelan diplomat to the United Nations on Thursday called on President Nicolas Maduro to resign immediately.

Isaias Medina said in an interview with Al Jazeera on Thursday that he decided to break with the government and resign shortly after a wave of political unrest began in early April - citing abuses during the demonstrations.

"The US and the European Union should act swiftly - not even wait if [Maduro] goes ahead or not with this national assembly," Medina told Al Jazeera.

"They should impose sanctions immediately; arms embargoes to begin with and of course without limit. We have to act together to free our country from this totalitarian regime that is trying to change the constitution illegally," he said.

Venezuela's Ambassador Rafael Ramirez said on Twitter that he "condemned the conduct of Isaias Medina" and that he had been immediately dismissed.