CARACAS (Reuters) - Three of Venezuela s largest opposition parties vowed on Monday to boycott mayoral polls later this year in protest at an election system they say is biased in favor of President Nicolas Maduro s ruling socialists. The multi-party Democratic Unity coalition has had a tough 2017, first failing to bring down Maduro in four months of protests that led to 125 deaths, then losing surprisingly to the Socialist Party in a gubernatorial election earlier this month.  That has left the opposition weakened and divided, and Maduro strengthened, despite growing foreign pressure on his government over alleged rights abuses and corruption, and an unprecedented economic crisis that has millions skipping food. Three heavyweight movements in the opposition - Justice First, Popular Will and Democratic Action - announced on Monday they did not trust the government-leaning election board sufficiently to participate in the municipal polls in December.  Justice First leader Julio Borges, who also heads the opposition-led congress, said authorities cheated in the 2013 presidential election, denied Venezuelans a recall referendum last year, and rigged the Oct. 15 gubernatorial vote.  So instead of going into another  manipulated  vote, the opposition should focus on demanding reforms to the election board in anticipation of next year s presidential poll, he said.  The objective remains getting Nicolas Maduro out of power, and in this struggle, the world is with us,  he told reporters.  To the surprise of some, the Democratic Action party also joined the boycott. Its candidates won four governorships in October s vote and then infuriated many opposition supporters by swearing loyalty to a pro-Maduro legislative superbody.  Opposition supporters have been split over participating in elections this year. Some say it is the only way to show they are a majority and undermine Maduro, while a growing number argue there is no point in fighting a  dictator  via a system rigged in his favor.  They are pinning their hopes on international action, including U.S. sanctions against Maduro s government.  Maduro, whose personal popularity has plunged since his 2013 election due to food shortages and runaway inflation, said  sabotage  and  insurrection  were being planned against the mayoral votes.  I declare myself in battle,  the 54-year-old successor to Hugo Chavez said in a meeting with governors on Monday.   Those who attack the election system must pay.  Opposition leaders say the government has long been rigging elections by gross abuse of state funds in favor of socialist candidates, and dirty tricks such the last-minute moving of vote centers in opposition areas for the October ballot.  They have also presented some allegations of ballot-rigging. However, Maduro insists Venezuela s system is entirely trustworthy and impossible to hack. It has received international praise in the past, although it was slammed over July s vote for the Constituent Assembly superbody.  Maduro says the street protests earlier this year were a mask for a U.S.-backed coup plot, and accuses opponents of wanting to oust him by undemocratic means.   Venezuelans want ballots, not bullets,  said Maduro. 