By Alexandra Ulmer

CARACAS (Reuters) - Streaming down from hilltop slums in the dead of night, hundreds of Venezuelans join an ever-growing line that circles the vast "Bicentennial" state-run supermarket.

By sunrise, there are several thousand, closely watched by National Guard soldiers, all waiting for the chance to buy coveted rice, flour or chicken at subsidized prices amid crippling nationwide shortages and inflation.

Many of them used to be devoted supporters of Hugo Chavez, the late socialist president who brought his quirky brand of left-wing nationalism to the OPEC nation during a 1999-2013 presidency.

Now, in the grumbling of pre-dawn lines, there is disillusionment with Chavez's "Beautiful Revolution" and undisguised anger at his successor and self-declared "son" Nicolas Maduro.

Word that no price-fixed food - only diapers, detergent and deodorant - would be on offer this particular morning spreads quickly, further deflating and frustrating the crowd.

The day before, when food ran out, there was a riot.

"This is unbearable," says Wilson Fajardo, 56, a mechanic whose three children ate only bread for dinner the previous night. "We voted for Maduro because of a promise we made Chavez, but that promise has expired. Either they solve this problem, or we're going to have to take to the streets."

It is these people - who struggle to find food or medicine amid worsening shortages, see their income gobbled up by runaway inflation, and suffer near-daily water and power cuts - who are arguably a bigger problem for Maduro than his formal opponents.

For sure, the opposition coalition is organizing marches and trying to channel discontent into a drive for a recall referendum against the former union leader and bus driver.

Yet they are failing to attract large numbers to protests and Socialist Party officials say the referendum will not happen this year, confident the government-leaning electoral body will drag its feet on the complicated paperwork needed.

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As institutional channels to remove Maduro close, anger is spilling over in other ways.

Small spontaneous demonstrations are picking up: about 17 per day around the nation, according to the Venezuelan Observatory for Social Conflict, a rights group. It says looting, too, is becoming more common, with 107 incidents in the first quarter.

In towns around the nation, it is becoming common for neighbors to block roads or gather near state utility offices to show their rage over power-cuts, food prices or lack of water. Videos of mobs breaking into shops, swarming onto trucks or fighting over products often make the rounds of social media.

"I don't know if we can keep going like this ... I'm always in a bad mood," says construction worker Juan Carlos Cuello, 32, holding his sleeping four-year-old son in the line outside the Bicentennial where he arrived at 3.30 a.m.

Around him, people swat mosquitoes, cradle babies and swap stories about skipping breakfast or surviving on plantains and yucca in what they now wrily call the "Maduro diet". Cuello, a former "Chavista," says his two children often go to school on an empty stomach and have lost weight.

SHOWDOWN

The month of May has not been kind to Maduro.

First, the opposition submitted nearly 2 million signatures to kick off the recall referendum - way beyond the roughly 200,000 required. Nearly 70 percent of Venezuelans want him gone this year, a poll recently showed.

Then, Brazil's leftist President Dilma Rousseff was impeached in Brazil, depriving him of another important ally in Latin America, where the right has made a comeback in recent elections.

And in the United States, intelligence officials told reporters they were preparing for a "meltdown" in Venezuela.

Outraged, Maduro responded by saying a foreign invasion was in the works and ordered a state of emergency and military exercises to protect the nation.

"Washington is activating measures at the request of Venezuela's fascist right, who are emboldened by the coup in Brazil," he said in one of his daily speeches to the nation.

The military, packed with Chavez loyalists, is seen as key to Maduro's survival.

There is speculation that the economic crisis and Maduro's rambling speeches are costing him support in the inner circle. Unlike his predecessor, Maduro was not a soldier, but he often appears at army parades and has tapped several members of the military as ministers.

He still enjoys the loyalty of a core among the population who adored his charismatic predecessor Chavez for tapping an oil bonanza to provide the poor with housing and pensions

The government claims that a malicious right-wing elite is hiding food to stoke unrest and gain control of Venezuela's oil reserves, the world's largest.

That message resonates among some in the food lines, where there are memories of a brief 2002 coup against Chavez and anger against Maduro does not necessarily equal support for the opposition.

"I'm only half eating... (but) it's not Maduro's fault! There's an economic war going on," says Maria Perez, 55, in a special line for elderly, pregnant and handicapped people at the supermarket in east Caracas, in the shadow of Petare slum.

"The opposition just wants to get power," she says of the referendum push, before repeating a familiar complaint that opposition leaders are elitist and out-of-touch. "When they were in charge, they didn't take the poor into account."

Despite a decisive victory in December's legislative election, the opposition remains shackled by that image problem and squabbles among the two dozen parties in the coalition.

Their struggle to mobilize Venezuelans and fears of more violence after 2014 protests that left 43 people dead may yet keep a lid on unrest.

"I don't feel people are that ready, or they wouldn't be here letting themselves be humiliated," says Carolina Briceno, 35, who joined the line after her shift at a night club.

"I can't stand Maduro. Of course I would protest. I don't know what we're waiting for."

(Additional reporting by Andrew Cawthorne; Editing by Kieran Murray)