- It is the first time in Botswana that an ex-president has abandoned the ruling Botswana Democratic Party.
- Thousands of people who thronged the meeting at the town's showgrounds also threw away their BDP membership cards.
- Masisi will contest his first election in October this year, which analysts warn will not be a tough run for the BDP.
Botswana's former president Ian Khama on Saturday quit the governing BDP which has ruled since independence more than half a century ago, citing a fall-out between him and his successor.
It is the first time in Botswana that an ex-president has abandoned the ruling Botswana Democratic Party (BDP) in the history of a 57-year-old organisation which has ruled Botswana uninterrupted since independence in 1966, Khama told thousands of supporters in his rural home town of Serowe that he made a "mistake" in choosing Mokgweetsi Masisi as his successor in the diamond-rich country which has enjoyed a carefully-crafted reputation for stable government.
"I came here today to tell you I am parting ways with the BDP. I am throwing away my BDP membership card," said Khama as he trashed his card.
"I don't recognise the BDP anymore".
Thousands of people who thronged the meeting at the town's showgrounds also threw away their BDP membership cards.
Khama, whose father Sir Seretse Khama was the founding president of Botswana serving from 1966 to 1980, said he did not have a party yet but will support the opposition parties fight the BDP.
Khama and his successor have fallen out publicly, with Masisi reversing some policies introduced by his predecessor - including most recently the lifting of a ban on elephant trophy hunting.
Botswana has a two-term presidential limit and in 2018, Khama had picked his then-vice president Masisi to succeed him after serving the maximum 10 years in office.
But Masisi has moved to break with the past and establish his own authority since his inauguration a year ago.
Masisi will contest his first election in October this year, which analysts warn will not be a tough run for the BDP.
The rift between the two was laid bare last March when Khama accused his protege of betrayal.
Khama on Saturday appealed to BDP members to vote for opposition parties in a bid to remove Masisi.
"I know that some of you are uncomfortable with throwing away your BDP membership but you could still vote for the opposition candidates so that we topple the ruling party," he said.
Black T-shirts emblazoned with Khama's portraits and captioned "E seng mo kgosikgolo" Tswana for "hands off our paramount chief" were distributed at the event.
Khama is a traditional chief in the Serowe area of eastern Botswana.
A labour consultant Gosa Rapelang, 35, travelled more than 200 kilometres (125 miles) from the city of Francistown to support Khama's decision.
"What Khama did is fair. The situation in the BDP is toxic," she said. "We are going with him whereever he is going."
But unemployed 30-year-old Goitsemodimo Ramadi calls for dialogue between the feuding camps.
"These people should sit down and resolve their differences. Nothing positive will come out of this situation," said Ramadi.
"It is not too late to resolve their issues. I am not sure if I will vote for opposition party in October. Time will tell".
Independent political analyst Anthony Morima said Khama commands "a huge following. You ignore him at your own peril".
If opposition parties enter into a coalition the ruling party will struggle to maintain its grip on power.
"These elections won't be easy for the BDP to win," said Morima
The BDP did not immediately respond to Khama's decision.