While eastern Santa Cruz is the heart of Bolivia’s economy, political power has traditionally rested in La Paz in the west, provoking tense regional rivalry in negotiations.

A decision by the military to deploy to the streets on the evening of Nov. 11 to quell the unrest forced the negotiators’ hands. No civilian official had ordered the military to deploy, which only underscored the danger of prolonging the power vacuum.

The negotiators agreed on Ms. Añez shortly before Mr. Morales’s plane took off for Mexico the evening of Nov. 11, Mr. Quiroga said.

“We got up from the table with a deal,” said Luis Vásquez, a former senator who was at the talks.

But when the opposition lawmakers gathered in Congress the following day to swear in Ms. Añez, the MAS lawmakers did not show up.

Opposition negotiators and one of the mediators at the talks said this broke the agreement. Another mediator said the lawmakers from Mr. Morales’s party had turned back at the last minute out of fear for their safety. Neither mediator wanted to be named because the political situation is still so delicate.

Left without congressional support, Ms. Añez pronounced herself the interim president on Nov. 12 by citing a Supreme Court ruling that recognized her as Bolivia’s de facto leader.

Holding a Bible , she told a crowd gathered under the balcony of the presidential palace, “My commitment is to bring back democracy and tranquillity to the country.”