By Diego Ore and Deisy Buitrago

CARACAS, Dec 2 (Reuters) - Venezuela's second massive power outage of the year plunged much of the nation into darkness on Monday night, prompting renewed talk of sabotage from President Nicolas Maduro's government and cries of incompetence from its foes.

Power went off in Caracas and other cities around the country soon after 8 p.m. local time (0030 GMT), to the intense annoyance of residents and commuters.

"I feel so frustrated, angry and impotent," said sales adviser Aneudys Acosta, 29, trudging through the rain along a street in the capital after having to leave the disrupted underground transport system.

"I live far away and here I am stuck under the rain. Something's going wrong that they're not sorting out. The government needs a Plan B. This is just not normal."

Monday's outage appeared similar to a massive Sept. 5 blackout that was one of the worst in the South American OPEC member's history.

Maduro, a 50-year-old former bus driver who narrowly won a presidential election this year after the death of his mentor and former leader Hugo Chavez, accused the opposition then of deliberately sabotaging the grid to discredit him.

His powerful ally and National Assembly president, Diosdado Cabello, repeated the same accusation after Monday's blackout.

"I have no doubt that today's electricity sabotage is part of the right-wing's plan," Cabello said on Twitter.

In some wealthier parts of Caracas, where opposition to the socialist government is strongest, people began banging pots and pans out their windows in a traditional form of protest.

Some shouted, "Maduro, resign!"

Venezuela has been suffering periodic electricity cuts around the country for several years, although the capital has been spared the worst outages.

Critics say the power problems symbolize the failure of the government and its 15 years of socialist policies in resource-rich Venezuela.

The country has the world's largest crude oil reserves and is blessed with big rivers that feed hydroelectric facilities generating two-thirds of its power.

The blackouts, sometimes due to planned power rationing and at others to utility failures, have not affected the oil refineries, which are powered by separate generator plants.

State oil company PDVSA said its installations were all working normally, and fuel supplies were guaranteed.

Electricity Minister Jesse Chacon said the same major transmission line that went down in September - and carries about 60 percent of national supply - had again been affected.

He said power would return to Caracas within an hour. Parts of the capital were slowly regaining power during the evening, according to residents.

"We ask Venezuelans for patience," Chacon said. "It's noteworthy that the problem is in the same place as the sabotage in September."

Maduro, who was giving a live address on state TV when the power went off, said he was continuing to work in the presidential palace despite the "strange" blackout.

"Nothing and nobody will stop the offensive to protect the people from the bourgeoisie's economic war," he said on Twitter, using similarly militaristic language as his predecessor Chavez.

Since winning office in April, Maduro has accused local political opponents of conniving with wealthy businessmen and their allies in the United States to undermine his government.

As well as saying they sabotaged the power grid, he has alleged that they plotted to assassinate him and want to destroy the economy through price-gouging and the hoarding of products.

Venezuelans are suffering from a 54 percent annual inflation rate, as well as scarcities of basic products from flour to toilet paper. Nationwide local municipal elections on Sunday are seen as a major test of Maduro's standing. (Additional reporting by Andrew Cawthorne and Daniel Wallis; Writing by Andrew Cawthorne)

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