LA PAZ, Bolivia — A bomb caused an explosion that killed four people and wounded 10 during Carnival celebrations, Bolivian authorities said Wednesday.

The blast occurred late Tuesday in the middle of Carnival celebrations in the city of Oruro, the capital of President Evo Morales' home province. Police Gen. Faustino Mendoza told reporters the bomb was made of dynamite, ammonium nitrate and fuel oil, all commonly used in homemade explosives.

Mendoza said the blast dug a crater nearly 4 feet (1.3 meters) wide and 1.5 feet (43 centimeters) deep.

On Saturday night, an explosion along the Carnival route two blocks from Tuesday's attack killed eight and injured 40. Police had blamed the first explosion on a food vendor's gas canister, but said they were re-evaluating that in the wake of the second attack.

"The proximity of one to the other raises doubts," Defense Minister Javier Zavaleta said.

Officials said three people had been detained for questioning in the second explosion.

An ally of Morales publicly implied that opposition forces were behind the attack, while opposition members urged government supporters to refrain from blaming anyone without evidence. Bolivia has seen very little political violence over the last 12 years and a politically motivated bombing would be highly unusual.

Morales' backers and opponents plan marches across the country Feb. 21, the two-year anniversary of a national vote rejecting the leftist president's plan to seek a fourth term. Despite the vote, Bolivia's Constitutional Court is allowing Morales' plans to go forward.

"If the explosion is an action by the ultra-right, what's underway is a conspiracy against democracy," Hugo Moldis, a former member of Morales' administration, said on Twitter.

Opposition Sen. Oscar Ortiz called on the government and its allies to refrain from making unfounded accusations.

"We don't want this to become a source of speculation," he said. "Let police and prosecutors carry out a responsible, credible investigation."