The troops were sent in after staff at Santa Cruz airport detained a US plane to demand that fees be paid locally rather than to the federal authorities.

Correspondents say security has worsened in recent weeks in Santa Cruz.

However, the Associated Press news agency reported that the troops had only withdrawn to a military area inside the airport as a conciliatory gesture, and that government officials had claimed the army was not backing down.

'Commander'

The protesters were responding to a call from the province's governor, Ruben Costas, for people to turn out in huge numbers on Friday to wrest back control of the airport.

"We need 20,000 to 50,000 people awaiting orders from the only commander in this town, who is me," said Mr Costas, one of the fiercest opponents of Bolivian President Evo Morales.

The gas-rich province is seeking autonomy from the federal government.

On Thursday Mr Morales defended the action of troops who had fired tear gas at hundreds of protesters as they tried to storm the airport.

He said it was unacceptable that airport workers had tried to force airlines to pay landing fees to the local, rather than federal, authorities.

Federal officials are running the Viru Viru airport in Santa Cruz and say flights are operating normally despite the protests.

The crisis began when airport workers held up an American Airlines plane bound for Miami on Tuesday, demanding that landing fees be paid on the spot rather than passed on to the federal aviation authorities.

Mr Morales said he had ordered the military intervention to prevent the airport losing its good reputation.

This in turn brought several hundred people to the airport, who tried to break through the gates but were forced back by the troops.

The airport "has been stolen by the government using army troops," one protester told AP.

The local airport authority used to appoint its own directors, but three months ago federal officials installed their own person to lead the agency, AP said.

The airport crisis is part of a long-running dispute between local leaders in Santa Cruz and the central government of President Morales.

The province has rich farmlands and is the centre of Bolivia's energy industry.

Santa Cruz leaders want autonomy from the central authorities and a bigger share of natural gas revenues.

They also oppose attempts by President Morales to nationalise key industries and redistribute land.