By Andrew Cawthorne and Anggy Polanco

CARACAS (Reuters) - Venezuela's opposition plastered election centers with slogans and rallied in honor of dead protesters on Monday in a final week-long push to force President Nicolas Maduro into aborting a controversial congress.

The unpopular leftist leader is pressing ahead with the vote for a Constitutional Assembly on Sunday despite the opposition of most Venezuelans, a crescendo of international criticism, and some dissent within his ruling Socialist Party.

Critics say the assembly, whose election rules appear designed to ensure a majority for Maduro, is intended to institutionalize dictatorship in the South American nation, a member of OPEC.

But Maduro, 54, whose term runs until early 2019, insists it is the only way to empower the people and bring peace after four months of anti-government unrest that has killed more than 100 people and further hammered an imploding economy.

Knots of opposition supporters gathered at various centers where Venezuelans will vote on the assembly to leave messages, chant slogans and wave banners. "It's preferable to die standing than to live on our knees!" said one poster at a Caracas school.

"They want to install a communist state in Venezuela, but we're tired of getting poorer and will stay in the street because we do not want the Constituent Assembly," said lawyer Jeny Caraballo, 41. "The people are saying 'No'!"

The opposition, which has now won majority backing after years in the doldrums during the rule of Maduro's predecessor Hugo Chavez, also held nationwide rallies in the afternoon in honor of protesters slain during the crisis. Fatalities have included opposition and government supporters, bystanders and security officials.

Mourners gathered in Caracas with rosaries, candles and flags but National Guard soldiers on motorbikes interrupted the ceremony by lobbing tear gas at the crowd.

"The National Guard represses us even when we're praying for our fallen ones," opposition lawmaker Delsa Solorzano said on Twitter.

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Later on Monday, rights group Penal Forum reported that lawyer Angel Zerpa, one of the new Supreme Court magistrates sworn in by the opposition in defiance of the government, had been charged with treason in a military court. Zerpa, who was first detained on Saturday, has gone on a hunger strike, Penal Forum director Alfredo Romero said.

48-HOUR NATIONAL SHUTDOWN

The Democratic Unity coalition has raised the stakes by calling a two-day national strike for Wednesday and Thursday, after millions participated in a 24-hour shutdown last week.

Young members of a self-styled "Resistance" movement said the moves by the formal opposition were not tough enough, and are threatening armed action. For months, youths have blockaded streets and used slingshots, stones, homemade mortars and Molotov cocktails to battle National Guard troops.

Soldiers have been shooting tear gas canisters straight at the protesters, and also using rubber bullets and water cannons to disperse them in near-daily running battles.

At the weekend, Maduro said his government was "ready for any scenario" and blasted his foes as "terrorists" servile to Washington. "We're not surrendering to anyone!" he said.

The government has declared election centers "zones of special protection" and planned to deploy more than 230,000 soldiers to keep the peace on Sunday. On Monday, National Guard troops pulled down posters at some election centers to shouts of "murderers" from opposition supporters.

With U.S. President Donald Trump threatening economic sanctions on Venezuela, potentially aimed at the oil sector accounting for 95 percent of its export revenues, Maduro said he could count on "great friends" like China and India if needs be.

But the threat of sanctions on an already vulnerable Venezuela has spooked investors. Venezuelan bonds dropped on Monday on fears about the vote and possible sanctions.

Many families have been stocking up on food in preparation for trouble and shops being closed during a tumultuous-looking week. "It's traumatic what we're going through, but if it means an end to this nightmare, it will all be worth it," said Nancy Ramirez, 33, lining up for rice at a store in Caracas.

Details have been scarce on what Maduro's Constituent Assembly would actually do, but it would have power to rewrite the national charter - written under Chavez in 1999 - and override all other institutions.

Officials have said it would immediately replace the existing National Assembly legislature where the opposition won a majority in 2015 elections.

Consultancy Teneo Intelligence said the Constituent Assembly would be unlikely to change economic policy or the government's approach to foreign debt. "This is primarily a political gambit to keep 'Chavismo' in power, not an ideological or policy pivot," wrote analyst Nicholas Watson, in reference to the ruling socialist movement founded by Chavez.

(Additional reporting by Fabian Cambero and Alexandra Ulmer; Additional writing by Alexandra Ulmer; Editing by Andrew Hay and Bill Trott)