By Alexandra Ulmer

SAN CRISTOBAL (Reuters) - Masked youths are once again blocking streets and burning tires in the Venezuelan city of San Cristobal, the epicenter of last year's massive anti-government protests.

The groups are small and the unrest contained, but dissent is rising in this volatile Andean city, a barometer of frustration with nationwide shortages that are putting pressure on the socialist government of Nicolas Maduro.

Students, who also accuse the government of corruption and repression but whom Maduro labels "coupsters," are threatening to unleash larger demonstrations again.

"It's time," Deiby Jaimes, 21, said from behind a barricade of burning trash as police gazed down from their hilltop perch. "There's a social, economic and political crisis. Economically we're completely lost and in a delirium."

But Jaimes and other students said they were restraining themselves to see if other Venezuelans also take to the streets.

Last year's protests split the opposition and failed to attract widespread support from Venezuela's poor, meaning mainstream anti-government leaders like Henrique Capriles are calling for less radical tactics including peaceful rallies and a good showing at an upcoming parliamentary vote.

"People are scared," said Jaimes, an accounting student, as dozens around him knocked rocks together menacingly. "But fear is disappearing due to shortages. We're expecting a social explosion."

High demand and a Christmas lull in distribution have aggravated shortages across the nation of 30 million people. Queues sometimes snake around entire blocks, prompting isolated scuffles for coveted milk or diapers.

Although there has been scattered violence around the OPEC nation, many eyes are once again on the opposition hotbed of San Cristobal, where clusters of demonstrators have been facing off with security forces since the New Year.

It was here that the attempted rape of a student last year prompted protests that spread into a wave of national demonstrations.

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Major General Efrain Velasco Lugo, who is in charge of security for the western Andean region, called the protesters misguided delinquents. "They want to torch the city again."

Their motto, he added, can be boiled down to "because I think differently to you, I'm going to topple you."

Indeed, Maduro says right-wing foes, encouraged by the United States and compliant foreign media, are plotting an "economic coup" to topple his socialist government. Protesters retort they are decrying flawed policies, like currency controls that have crimped imports and led to shortages.

Army officials said on Thursday 18 protesters had been arrested in San Cristobal, capital of Tachira state, in the last 10 days, with six currently behind bars.

Rights group Penal Forum said 56 demonstrators were arrested nationally this year, with most now released.

A national guard shot a protester in the chest on Thursday night during clashes in San Cristobal, a student leader said. Reuters could not immediately verify the information.

The situation remains a far cry from unrest between February and May that left 43 dead and hundreds injured during the biggest disturbances in more than a decade. Victims included demonstrators, government supporters and security officials.

COMBATIVE 'CORDIAL CITY'

Still, the mood is increasingly combative in San Cristobal, traditionally known as the "cordial city," as life becomes a series of queues.

Taxi driver Luis Perez wakes up around 5 a.m. to wait in line for gasoline.

"We produce so much oil, and look how we're suffering," he said as he finally filled up his creaking blue 1982 Chevrolet.

"We need a change of government," he added before paying less than 2 cents a liter for the world's cheapest gasoline.

Roughly 15 percent of fuel in Tachira is smuggled out of the state, estimates Nellyver Lugo, a ruling party state legislator who heads a commission on gasoline. Lack of spare parts for trucks and tricky contract negotiations reduced supplies this year, she added.

Up to 25 percent of food is smuggled out for sale at a hefty profit in Colombia, the army says, citing discoveries of subsidized flour stashed in tires or rice in engines.

Even once-fervent "Chavistas" are becoming skeptical as inflation and shortages threaten anti-poverty advances under the late Hugo Chavez's 1999-2013 rule.

"There was a lot of hope, but things didn't pan out the way we wanted," Ronald, a government employee who would not give his last name, said as he stood in line clutching scarce toilet paper. "Now we're paying the price. I hope they implement changes."

But Maduro, whose approval levels have steadily eroded since his 2013 election, has so far balked at implementing pressing but unpopular measures such as raising gasoline prices or unifying a baffling three-tiered currency control system.

Sinking oil prices have compounded Venezuela's cash crunch, prompting fears that the nation may have to default. An impending national parliamentary election has raised the stakes further.

With Maduro out of the country for the last 10 days on an apparently unsuccessful trip to lobby for an oil supply cut, Venezuela's perennially fragmented opposition is scrambling to unite and call for peaceful protests.

"The government is weaker," 24-year-old student leader Reinaldo Manrique said, standing next to a charred bus near the University of the Andes.

"It won't survive an explosion like last year's."

(Writing by Alexandra Ulmer; Editing by Andrew Cawthorne and Lisa Von Ahn)