LA PAZ, BOLIVIA - Bolivia's public prosecutor has opened an investigation into "electoral fraud" against former president Evo Morales and some of his closest allies, the attorney general said on Wednesday.

"A new case has been opened against the ex-authorities for electoral fraud," said Juan Lanchipa.

Morales was controversially awarded victory at October's election despite a highly suspicious 24-hour freeze in the live vote count, after which his lead over nearest challenger Carlos Mesa had jumped significantly.

An audit by the Organization of American States found clear evidence of vote rigging.

After three weeks of at times violent protests against Morales's re-election, the 60-year-old resigned on November 10 and fled abroad.

October's election was annulled a new one called for May 3 by interim President Jeanine Anez.

Former president Mesa brought the case against Morales, accusing him of falsifying and altering documents, blocking electoral processes and concealing results.

Following Morales's resignation, the six members of the electoral tribunal were arrested. Five are currently in jail and the sixth under house arrest.

Mesa says the investigation against them is inconclusive because it "doesn't take into consideration the intellectual authors" of the fraud.

The case against Morales and his associates -- former vice president Alvaro Garcia and ex-ministers Juan Ramon Quintana, Hector Arce and Carlos Romero -- has been brought so that "never again a president of the state, abusing his power, makes a mockery of the public's vote," said Mesa's lawyer, Carlos Alarcon.

He said it was "inadmissible" that only the six magistrates had been investigated when "they didn't act for their own benefit."

"They acted for the benefit of the ex-president Morales and the leadership of his government."

Morales, Bolivia's first indigenous president who ruled for almost 14 years, is already under investigation for sedition and terrorism over an audio recording in which he allegedly urges his supporters to lay siege to La Paz and other major cities.

The former trade union leader has been barred from standing for president in May's general election -- Bolivia's constitution limits a president to two successive terms, while Morales ruled for three in a row.

His Movement for Socialism party has nominated him to stand for a berth in the Senate but the electoral tribunal -- which has been entirely replaced since he left power -- is studying his eligibility.