By Daniel Wallis and Andrew Cawthorne
CARACAS, Feb 26 (Reuters) - U.N. Secretary-General Ban Ki-moon and Pope Francis called on Wednesday for an end to violence in Venezuela that has killed at least 13 people and urged politicians to take the lead in calming the nation's worst unrest in a decade.
Students and other opponents of President Nicolas Maduro are demanding that he quit over grievances including high inflation, shocking levels of violent crime, shortages of basic food, and what they say is his repression of political rivals.
The protests are the biggest challenge to Maduro's 10-month-old administration, although there is no sign they could topple him or affect the OPEC nation's oil shipments.
Among the latest world figures to speak out about the unrest, Pope Francis told tens of thousands of people in St. Peter's Square he was "particularly concerned" by recent events.
"I sincerely hope the violence and hostility ends as soon as possible, and that the Venezuelan people, beginning with the responsible politicians and institutions, act to foster national reconciliation through mutual forgiveness and sincere dialogue," Francis said during his weekly address.
U.N. head Ban called for "concrete gestures by all parties to reduce polarization" and engage in dialogue. "He appeals to Venezuelans, no matter their political perspective, to voice differences and grievances peacefully and in accordance with the law, and to seek common ground," a statement added.
Maduro, a 51-year-old former bus driver and union boss, hosted church and business leaders and some opposition politicians for a "national peace conference" on Wednesday night.
Among the attendees were Jorge Roig, president of Venezuela's main business chamber, and Lorenzo Mendoza, the billionaire head of the nation's largest private company Polar. The government has frequently excoriated both as heartless capitalists leading an "economic war" against Maduro.
"The country is sick, Mr. President," Roig told Maduro, defending protesters' grievances and criticizing the government's "failed" economic model in one of a series of short speeches by attendees broadcast by state television.
CAPRILES BOYCOTTS MEETING
The most prominent opposition figures declined to attend, saying Maduro was using the meeting as smokescreen to avoid tackling Venezuela's real problems.
"This cannot just be a photo op," two-time opposition presidential candidate Henrique Capriles told Reuters.
"Who does dialogue suit more? Nicolas, I think ... This is a government that is becoming extinct, eating itself up."
Earlier, female opposition supporters donned white clothes to march in silence from a western Caracas neighborhood to a nearby National Guard military base, carrying photographs of victims of alleged brutality by the security forces.
In a rival rally, pro-Maduro farm workers clad mostly in the bright red of the ruling Socialist Party marched in downtown Caracas under the slogan, "Sowing peace and harvesting life!"
Opposition demonstrations began at the start of the month, and mushroomed when three people were shot dead on Feb. 12.
Video and photographs taken on the day showed men widely believed to be state security agents apparently firing pistols at stone-throwing student protesters clashing with police.
Five intelligence agents have been detained over two of the deaths, suspected of crimes including homicide.
Maduro, who narrowly won a presidential vote in April to replace the late Hugo Chavez, accuses foreign media of working with "imperialists" abroad to project an image of chaos.
TENSE IN TACHIRA
About 150 people have been injured during the two-week crisis, and more than 500 people arrested. Fifty-five people remained behind bars on Wednesday, including 11 security officials held for presumed rights abuses, the government said.
Maduro said on Wednesday more than 50 people had died from the unrest. He was referring not only to the 13 people shot or directly killed around protests and rallies, but also those indirectly affected by, for example, being blocked from getting to hospital.
"An 84-year-old lady died in east Caracas yesterday because they had her held up in a street blockade for more than three hours," he said. "She died in her family's car."
The worst of the violence is centered on the western state of Tachira, bordering Colombia, where officials in several municipalities reported the looting of a supermarket, clothes shops, discos and other businesses overnight. Several people were hurt by plastic buckshot fired by security forces.
Ricardo Hernandez, mayor of Cardenas municipality, close to the state capital San Cristobal, blamed the looting on hooded motorcyclists, and said he had needed to call on reinforcements from the national police and National Guard troops.
"Take the necessary precautions, such as having telephones close to hand, doors tightly closed," Hernandez exhorted residents. "Keep bells or whistles nearby in order to alert the neighbors."
Farmers in Jauregui municipality, a major supplier of vegetables to the rest of the country, said they have 15,000 tonnes of produce they have not been able to send to markets because of the insecurity and barricaded highways.
Shortages are particularly acute in many areas of Tachira, where blocked roads and the threat of violence mean delivery trucks have not reached stores for days, residents say.
Moderate opposition figures have called for peaceful protests only and voiced despair at the tactics of barricading streets and burning trash in mostly middle-class neighborhoods that are already overwhelmingly pro-opposition.
Venezuelans are approaching a long weekend for Carnival, when families typically head to the beach. Possibly with an eye to taking the heat out of the protests, Maduro extended the break by also declaring Thursday and Friday national holidays.