(Updates throughout with strike start)
CARACAS, July 20 (Reuters) - Many Venezuelan streets were deserted and barricaded on Thursday for a strike called by foes of President Nicolas Maduro to demand a presidential election and the abandonment of a plan for a new congress they fear would institute a dictatorship.
Many businesses were staying shut as the opposition organized a 24-hour national shutdown from 6 a.m. in a civil disobedience campaign they are dubbing "zero hour" to try to end nearly two decades of Socialist Party rule
"There's no movement. Everything is closed. We must all do our best to get rid of this tyrant," said Miguel Lopez, 17, holding a homemade shield emblazoned with "No To Dictatorship!" as he manned a roadblock on a Caracas street devoid of traffic.
Many private transport groups heeded the strike call, while students, neighbors and activists hauled rubbish and furniture into streets to form barriers. Many families were keeping children behind doors in case of trouble.
Four months of anti-government unrest have killed about 100 people, injured thousands and left hundreds in jail.
Leaders of Venezuela's 2.8 million public employees said state businesses and ministries would remain open, and there was a trickle of workers on the street.
"I'm on strike 'in my heart' because if we don't turn up, they will fire us," said a 51-year-old engineer at state steel plant Sidor in southern Bolivar state, waiting at dawn for transport provided by her company.
No disruptions were expected at oil company PDVSA which brings in 95 percent of Venezuela's export revenue.
"VENEZUELANS WANT TO WORK"
Some people grumbled that the opposition action would cost them money and prevent them seeking food at a time of extreme economic crisis and hardship in the OPEC nation.
"Most Venezuelans want to work," Maduro's son, also called Nicolas and a candidate for the controversial Constitutional Assembly super body, said on the eve of the strike.
"I'm sure it will fail."
With Venezuela already brimming with shuttered stores and factories amid a blistering four-year recession, even a successful strike would have limited financial impact.
Security forces fired tear gas at protesters manning barricades in some places early on Thursday, witnesses said.
Clashes have been occurring daily since the opposition Democratic Unity coalition and a self-styled youth-led "Resistance" movement took to the streets in April.
Two hours into Thursday's strike, it was looking more successful than a similar action last year, which had a lukewarm impact after the government had threatened to seize any closed businesses. Since then, the opposition has gained momentum.
Maduro also faces widespread foreign pressure to abort the Constitutional Assembly, which could rewrite the constitution and supersede other institutions. Officials say it will replace the current opposition-led legislature.
U.S. President Donald Trump weighed into the dispute this week, threatening economic sanctions if a July 30 vote for the assembly goes ahead. The opposition is boycotting the vote, whose rules seem designed to guarantee a government majority despite its minority popular support.
Recalling a 36-hour coup against his charismatic and far more popular predecessor Hugo Chavez, the Venezuelan president has said his foes are seeking to oust him by force.
As well as a presidential election, Venezuela's opposition is also demanding freedom for more than 400 jailed activists, autonomy for the legislature and foreign humanitarian aid. (Additional reporting by Andreina Aponte in Caracas, Franciso Aguilar in Barinas, Anggy Polanco in San Cristobal, Maria Ramirez in Ciudad Guayana; Editing by W Simon)