The protesters died during pitched battles on Sunday with troops clearing roadblocks in the poor industrial suburb of El Alto, outside La Paz, human rights officials said.
The government, which has understated tolls in recent protests, said four civilians and one soldier were killed and about 30 others were injured.
Sunday's clashes raise the bodycount to around 30 dead with dozens injured during month-long protests against Gonzalo Sanchez de Lozada's free market policies and failure to tackle crushing poverty.
"It is difficult to come up with an exact toll, but according to the reports we have studied, there are 20 dead and 91 injured in or near El Alto," said Waldo Albarracin, president of Bolivia's Permanent Human Rights Assembly.
Fuel and basic foods were running short in the capital as thousands of poor Bolivian farmers and workers, calling for Sanchez de Lozada to quit stopped convoys of trucks entering the Andean city with roadblocks.
Presidential spokesman Mauricio Antezana said the government could order a curfew in El Alto at any time to stop what it perceived as a coup attempt by its opponents.
Witnesses said troops stood guard on the main road in El Alto, the centre of recent protests against the government.
Sunday's violence is the worst since February, when a government austerity drive, backed by the International Monetary Fund, sparked massive riots in which 32 people died.
Sanchez de Lozada has failed to
alleviate poverty in Bolivia
Two people were killed on Saturday and dozens more were injured as protesters fought pitched battles with police and security forces outside the capital, media reported.
Protests by the country's poor Indian majority against Sanchez de Lozada have spiralled in the last month amid an economic downturn in one of the poorest nations in the western hemisphere.
Indian leader and lawmaker Evo Morales, who nearly won the presidency in 2002, rejected the government's claims of a coup attempt.
"They (the government) are the subversive ones who are trying to act like coup leaders," he said.
Sanchez de Lozada, a US ally in the anti-drug war who is widely unpopular for failing to alleviate poverty, has played down the protests and defied calls to step down.
Next week, transport workers and coca farmers are expected to join the protests.
They are angry at a US-backed drive to eradicate illegal crops of coca, the raw material used to make cocaine.