He accused Leso of making allegations against him without any evidence.
The party polls, which ended on Wednesday morning, were seen to determine the direction of the party, with the presidential election due in October this year.
Mbowe has acknowledged the loss, posting on his X account that he “fully accepts the decision of the party assembly” with a photo of him and Leso.
“I congratulate Tondo Leso and his colleagues for being trusted to take over the leadership of the party. I wish them all the best in taking our party forward,” he said.
In a closely fought election, Leso received 513 votes (51.5%) to Mbowe’s 482 (48.3%).
As he was declared the winner, Leso said the party had set the benchmark for internal democracy and called on other political parties in the country to follow his example.
Leso is known for his fiery brand of politics that often gets him into heated arguments with fellow politicians, especially those from the ruling party.
He survived an assassination attempt in 2017 during the administration of President John Magofuli. After being hit by at least 16 bullets, he underwent more than 20 operations in hospitals abroad.
He returned briefly in 2020 but lost to Magofuli in that year’s presidential election.
Leso then spent years in exile until he returned in 2023, when President Samia Salohu Hassan took over after Magofuli’s death in 2021.
He has since accused President Samia of a Magufuli-era crackdown on critics.
He has also spoken out strongly against human rights violations and recent kidnappings and disappearances of civilians, including opposition officials.
Leso himself and other opposition politicians have been arrested in recent months during a government crackdown on opposition protests.
During the party contest, he established himself as a fearless leader and his victory is being seen as a shift in the party’s direction to mount a strong challenge to the government as the country moves towards the end of this year. I am moving towards elections.
On Sunday, President Samia was unanimously nominated as the presidential candidate by his ruling CCM party.
Chidima has not yet chosen his candidate.
Tanzanian analyst Nicodemus Minde told the BBC that it was likely that Lissu or Mbowe would be the party’s flag-bearer if there were no results after the party’s election.
