Lowassa becomes the highest profile figure from the ruling party to join the opposition camp in Tanzania, which has east Africa's second-biggest economy.
The Chama Cha Mapinduzi (CCM) party has ruled since independence in 1961. The fractious opposition was initially not expected to pose a serious threat to its position in parliamentary and presidential elections on Oct. 25.
But analysts said Lowassa's defection could see Chadema, the main opposition party, mount a realistic challenge to CCM for the first time ever.
"I have decided to leave CCM ... and join Chadema," Lowassa told a news conference in Tanzania's commercial capital Dar es Salaam. "It is obvious that CCM has lost its direction."
Opposition sources said the main opposition coalition was expected this week to name Lowassa, a popular politician with a reputation for getting things done, as its presidential candidate.
Lowassa accused President Jakaya Kikwete's CCM of being undemocratic, saying it had unjustly removed his name from a short list of presidential candidates. Kikwete steps down this year at the end of his second and final term in office.
Tanzania's ruling party on July 12 named Works Minister John Magufuli as its candidate for this year's presidential race.
From a final list of three, Magufuli, 55, beat two female contenders - former senior U.N. official Asha-Rose Migiro and Amina Salum Ali, the African Union ambassador to the United States.
Lowassa, 61, was seen as a front runner in the ruling party's presidential nomination race but was not included in the short list of candidates, causing divisions in the party.
He quit as premier in 2008 over corruption allegations that he denies.
Tanzania has been one of Africa's most politically stable nations and has not been torn by debates raging in parts of the continent, where some presidents have sought third terms despite constitutional restrictions.
The main opposition parties promised last year to field a single candidate in the election, but experts say they may struggle to overcome years of mutual suspicion and infighting.
--reuters--
