Security forces fired tear gas at protesters
A chief coalition partner of embattled Bolivian President Gonzalo Sanchez de Lozada has withdrawn its support for the government amid continuing street protests.
The New Republican Force (NFR) announced it would severe its ties, as a popular uprising against government plans to export natural gas intensified.
Manfred Reyes Villa, head of the centre-right NRF, made the announcement as tens of thousands of people marched through Bolivia's administrative capital, La Paz, demanding the president's resignation.
The decision strips the president of his working majority in Congress.
The demonstration was one of the biggest protests since the crisis over the gas exports began more than a month ago, with around 50,000 people taking part.
These included indigenous groups, farmers, workers and miners.
Dozens have been killed and injured in the protests
Speaking after a meeting with the president, Mr Reyes Villa said: "There's nothing left to do but leave (the government). This can't go on."
He said the president had told him he would not resign, despite a revolt that has left at least 65 people dead.
Mr Sanchez de Lozada's vice president, Carlos Mesa, has already withdrawn his support for the president's policies and Minister of Economic Development Jorge Torres Oblea has resigned.
The protests have left much of La Paz paralysed after weeks of blockades; there is little food or fuel, pharmacies are running out of basic medicines and there is no money in cash machines, Reuters news agency reported.
The Bolivian president has already been forced to postpone the controversial gas export scheme until 31 December.
But the opposition has rejected his offer to hold a referendum on the project, saying it would accept nothing less than his resignation.
The BBC's Elliott Gotkine in Bolivia says the protests are not just about gas.
Bolivia is South America's poorest nation and the peasants want land reform, the elderly want better pensions and the workers want more money, he says.