Buenos Aires/La Paz  -  A Bolivian power struggle between the poor, indigenous majority and the wealthier people of European origin in the country's east comes to a head on Sunday as the wealthy province of Santa Cruz de la Sierra votes on a referendum seeking greater autonomy.

The electoral authorities have deemed the referendum illegal, and have warned they will neither monitor nor acknowledge its results, but opinion polls estimate that more than 70 per cent of the voters in Santa Cruz will cast a ballot favouring more independence.

On Saturday, supporters of leftist President Evo Morales burned ballot boxes in Yapacani and San Julian in the first sign of potential conflict over the vote.

The issue centres over control of profits from the area's rich natural resources and has been brought to a head in the ongoing nationalization of the country's energy resources.

The issue is so heated that observers are not prepared to rule out a break-up of the Andean country of 9 million people, squeezed between Brazil, Argentina, Chile and Peru, over the conflict.

Tension has been growing since Morales, a leftist former coca growers' leader, was elected in late 2005, becoming the country's first indigenous president. He began nationalizing the country's energy resources two years ago to fund government programmes in support of the Indio majority, who live mostly in the resource-poor highlands in the western part of the country.

Ahead of the vote, the government on Thursday announced it had taken over four international energy companies and would also take control of a telecommunications firm.

The referendum is to be followed by two others seeking greater autonomy in other provinces. (dpa)