(Updates throughout, adds byline, changes dateline)

By David Mercado

YACUIBA, Bolivia, April 19 (Reuters) - Bolivia's fuel exports were uninterrupted on Thursday even though demonstrators remaining in control of a key plant that pumps natural gas to Argentina.

Protesters demanding a share of taxes from a large Bolivian natural gas field freed some 50 police officers that had been held hostage for over 20 hours.

Earlier in the week one demonstrator was killed and several injured in clashes with police and troops.

Despite the presence of hundreds of troops sent by the government to guard key natural gas facilities, protesters are still in control of the Yacuiba pumping plant operated by Transredes, a Royal Dutch Shell Plc <RDSa.L> subsidiary.

A spokeswoman for Transredes told Reuters that company officials had to "run for their lives" and that protesters looted the site, destroyed documents and set several vehicles on fire.

An official from a government agency that monitors Bolivia's energy production told Reuters the plant continues operating despite the protest.

The leftist government, which nationalized the energy industry a year ago, appealed for calm and invited protesters to a meeting in the country's administrative capital La Paz on Friday.

Meanwhile, some 30 miles (50km) from Yacuiba, hundreds of demonstrators gathered outside the San Alberto natural gas field, run by Brazil's Petrobras, threatened to storm the site if President Evo Morales does not travel to Tarija to negotiate with them, local radio reported.

Tarija, on the border with Argentina, is home to 85 percent of the country's natural gas reserves, and energy majors including Petrobras <PETR4.SA>,Spain's Repsol YPF <REP.MC>, Brazil's and France's Total <TOTF.PA> operate natural gas fields there.

BIG TAX TAKE AT STAKE

The protests stem from a dispute between the neighboring provinces of Gran Chaco and O'Connor over who has rights to potential taxes from the Margarita field, which holds some 10.5 trillion cubic feet of natural gas.

Both claim the vast field, discovered in 2003 along their poorly defined boundary, is in their jurisdiction and demand a share of the taxes the state will receive once the field is producing and exporting natural gas to Argentina.

Repsol-YPF is the operator of the field and owns a 37.5-percent stake in the project, while British Gas Plc. <BG.L> also holds a 37.5 percent stake. Argentina's Pan American Energy holds the remaining 25 percent share.

Morales nationalized the natural gas-rich energy industry a year ago forcing foreign operators to pay much higher taxes and vowing to alleviate poverty with the extra funds.

(Additional reporting by Carlos Quiroga and Eduardo Garcia in La Paz)

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