LA PAZ — A lead opponent of Bolivian President Evo Morales has vowed to hand him a resignation letter to sign, raising pressure on the leftist leader over claims he rigged his re-election.

More than two weeks since the vote result sparked angry protests, Santa Cruz region opposition leader Luis Fernando Camacho flew to the capital La Paz to press Morales to quit.

Camacho, who was joined in La Paz by former president Jorge Quiroga and defeated opposition candidate Carlos Mesa, said he planned to deliver the letter to Morales' government headquarters.

Protesters blocked streets in La Paz, including the central Avenida Camacho a few blocks from the government headquarters.

The powerful Catholic Church cautioned against such an open challenge to Morales' authority however.

"Asking for the president's resignation is a radical measure," said Sucre Archbishop Jesus Juarez.

In the central city of Cochabamba, violent clashes broke out between supporters and opponents of Morales, leaving 20 people injured, some seriously, Ombudsman Nadia Cruz said.

"We have 20 people injured, some of the injuries are almost lethal," Cruz said.

Rival groups fought pitched battles in the city with rocks and sticks. Some protesters launched firecrackers from home-made bazookas. The town hall in neighboring Vinto, a stronghold of Morales' ruling MAS party, was burned down.

The Cochabamba clashes took place after members of farmers' unions, mainly women, began an effort early in the day to clear opposition roadblocks that have stopped traffic for several days.

"Evo, friend, the people are with you!" supporters chanted before clashing with opponents, mainly students, in a city square.

Anti-Morales protests were also held on Wednesday in the cities of Santa Cruz, Sucre, Tarija and Potosi, shutting down state offices and companies.

Bolivia's state oil company warned of likely fuel shortages because of the protests. "It is impossible to supply service stations," it said in a statement.

Camacho, 40, a conservative opposition leader in the eastern Santa Cruz region, has threatened to drive Morales out and called for the military's backing.

Morales addressed a navy ceremony on Wednesday, insisting that the military must "serve the people" and support his government.

The armed forces have so far stayed neutral in the electoral dispute, but calling on the military is a controversial move in Bolivia. — AFP