(Adds policeman's death, Maduro quotes)
By Andrew Cawthorne and Eyanir Chinea
CARACAS, April 10 (Reuters) - President Nicolas Maduro was due to host Venezuelan opposition leaders on Thursday at the start of mediated talks intended to stem two months of political unrest that has killed dozens in the OPEC nation.
Underlying the gravity of the crisis, authorities said on Thursday a policeman was shot dead in western Barquisimeto city while dispersing a demonstration.
That took the official death toll to 40.
Some hardline opposition groups, including the party of jailed protest leader Leopoldo Lopez, are boycotting Thursday's scheduled talks while dozens of demonstrators remain in jail.
The meeting, brokered by foreign ministers from the Unasur bloc of South American governments, was due to start at 5 p.m. (2130 GMT) at the Miraflores presidential palace in Caracas and be broadcast live on state TV.
Both sides have asked the Roman Catholic Church to attend as a "good faith" witness. The Vatican has confirmed its willingness to mediate but has not named an envoy to the talks.
"Miraflores will tremble," said opposition leader Henrique Capriles, who narrowly lost to Maduro in last year's election to replace late socialist leader Hugo Chavez.
"We will tell the government the truth so that the country can open its eyes and we see that things must change," added Capriles, who is part of the Democratic Unity opposition coalition's delegation to the talks.
Pollsters say approval levels for both Maduro and the opposition have fallen during the crisis, while an already slowing economy has suffered a further drag from the impact of violent clashes on businesses and transport.
Since protests began in early February, about 650 people have been injured in addition to the 40 fatalities, officials say. More than 2,000 people have been detained, and 174 are still behind bars.
MADURO: 'I WON'T BETRAY REVOLUTION'
Maduro, who calls himself the 'son' of Chavez and is seeking to preserve popular oil-funded welfare policies while tinkering with his predecessor's statist economic model, said ahead of the talks that he would talk but not negotiate.
"There will be no pact with anyone. There is a debate, a dialogue, which is different. I would be a traitor if I start negotiating the revolution," he said.
Hardline protesters have openly sought to provoke a "Venezuelan Spring" that would force Maduro out of office but have failed to bring the millions onto the streets they had hoped for. The demonstrators have proved persistent, however, with sporadic roadblocks, marches and other protest tactics in some cities.
Students in Caracas planned to rally just before the talks.
Maduro said if the opposition wants to get rid of him, it would have to be via the ballot only. His foes could force a presidential 'recall referendum' in 2016 if they garner about 4 million signatures for it. Maduro's six-year term ends in 2019.
"The opposition must return to the democratic and constitutional track," Maduro told thousands of supporters during a speech in a Caracas convention center, to cheers of "they will not return".
Venezuelans across the political spectrum are fed up with violent crime and economic problems including a 57 percent annual inflation rate and shortages of basic products from milk and flour to toilet-paper and car batteries.
Both sides said those issues would figure high in the talks.
Maduro said he had "positive surprises" for the opposition at the talks, but gave no details. There have been calls for a goodwill release of former security official Ivan Simonovis, who is serving a 30-year sentence for some shootings during a brief coup against Chavez against 2002. (Additional reporting by Deisy Buitrago; Editing by James Dalgleish)
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