President Carlos Mesa refuses to sign the bill into law because he considers the level of taxes unworkable.

The law, already approved by Congress, would raise the taxes paid by foreign energy firms to 50% of their revenues.

President Mesa is backed by business groups, but opposed by peasants, coca farmers and trade unionists, who want the bill to be made stricter.

The president has until Tuesday to make a final decision, but the BBC's Elliott Gotkine in La Paz says he is looking increasingly isolated.

Testing week

Trade unions, landless peasant farmers and neighbourhood groups are demanding the nationalisation of Bolivia's energy industry.

Thousands headed to La Paz from the sprawling satellite city of El Alto.
Some waved the rainbow-coloured flag of Aymara nationalism and others carried effigies of Mr Mesa.

Our correspondent says they see him as a stooge of the foreign energy companies working in Bolivia.

They say that nationalisation of the gas industry is the only way they will benefit from Bolivia's greatest natural resource.

Elsewhere in the country coca farmers, miners and supporters of opposition leader Evo Morales began a 200km (125 miles) march to La Paz.

Our correspondent says this week will be one of the most testing for Mr Mesa since he came to power 18 months ago.

He is expected to make some amendments to the bill before sending it back to Congress for final approval.

Our correspondent says this should reduce the sense of uncertainty in Bolivia, but it is unlikely to be the end of the president's problems.

Plans to export the country's gas sparked a wave of deadly protests in October 2003 and resulted in President Mesa's predecessor being forced from office.