The Bolivian opposition Senator Jeanine Áñez declared herself the interim president of the South American country following the resignation of Evo Morales.
Legislators of the Morales party boycotted the meeting, meaning that there is no quorum for the appointment.
But Ms. Áñez said she was next in line with the Constitution and promised to hold elections soon.
Mr. Morales condemned the announcement and described Mrs. Áñez as "a far-right coup d'état."
The former president fled to Mexico asking for asylum because his life was in danger.
He resigned on Sunday after weeks of protests against a controversial outcome of the presidential election. He said he was forced to resign, but he volunteered to "stop bloodshed."
How did the senator become interim president?
Ms. Áñez temporarily took control of the Senate on Tuesday and put her in the next row for the presidency.
The former Deputy Senate Chairman took over the position after a series of resignations.
While the legislators of the "Movement for Socialism" party of Mr. Morales were absent, Ms. Áñez declared herself provisional chairman.
On Twitter, Mr. Morales condemned the "most insidious, most shameful coup in history."
How did we get here?
Mr. Morales, a former coca farmer, was first elected in 2006 as the country's first leader of the indigenous community.
He won praise for combating poverty and improving the Bolivian economy, but was controversial by opposing constitutional boundaries to apply for a country's fourth term in the October election.
The pressure on him had increased since his narrow victory last month.
The result was questioned by the Organization of American States, a regional organization that found "unequivocal manipulations" and demanded the annulment of the result.
In response, Mr. Morales approved new elections. His main rival, Carlos Mesa, who finished second in the poll, said, however, that Mr Morales could not compete in a new poll.
The Chief of the Armed Forces, General Williams Kaliman, then called on Mr. Morales to lay down his interests of peace and stability.
When Morales announced his resignation, he said he had made the decision to prevent other socialist leaders from being "harassed, persecuted and threatened". He also called his removal a "coup".
He fled to Mexico when riots erupted on the streets of the Bolivian administrative capital of La Paz. The socialist leader clashed with security forces.
After arriving in Mexico City on Tuesday, he thanked Mexican President Andres Manuel Lopez Obrador, to whom he had attributed the salvation of his life.
"While I have life, I stay in politics" The fight continues. All people in the world have the right to free themselves from discrimination and humiliation, "he said.