The historic Botswana Democratic Party (BDP) -- whose rule has been unfettered since independence from Britain in 1966 -- was shaken up earlier this year after former president Ian Khama dramatically renounced his hand-picked successor Mokgweetsi Masisi.
Voters started gathering late on Tuesday at Tlogatloga secondary school in Botswana's capital Gaborone, waiting all night to cast their ballot.
"I have spotted a candidate I think is the right person and I have to get him in," said banker Chops Maswikiti, 37.
"He does not belong to the party I voted for in the last two terms but he represents strong sanity on our side," he told AFP.
Khama stormed out of the BDP in May after accusing President Masisi -- his deputy until last year -- of autocracy.
The dispute could fracture the BDP and offset Botswana's reputation as a beacon of democracy and stability in Africa.
It also strikes another blow to Botswana's founding party, which saw its share of the vote dip below 50 percent for the first time at the last election in 2014.
Khama has thrown his weight behind the BDP's strongest rival -- a coalition of opposition parties called the Umbrella for Democratic Change (UDC).
While the opposition group was once Khama's fiercest critic, he has urged voters to back the UDC in a bid to unseat the "dead" BDP.
Khama, whose father co-founded the BDP and served as Botswana's first president, retains plenty of influence, particularly in the central region -- a BDP stronghold -- where he is a traditional chief.
Should Khama fulfil his goal, it would be the first time diamond-rich Botswana has seen a change of government in 53 years.
Around 931,000 of the country's 2.2 million people are registered to vote in the parliamentary and local elections.
The BDP, UDC and two smaller parties are vying for 57 seats in parliament. The party with the most seats chooses the president.
"This election is different because normally it's obvious who will be the next president," said Lesetamang Paya, a 63-year old retired public servant, before casting his vote.
"But this time it's uncertain, there is genuine competition," he added, hinting that there "might be some change".
'Rule of law remains supreme'
During an interview with AFP in the town of Mahalapye, 200 kilometres (120 miles) northwest of Gaborone, Khama said the "arrogant" Masisi administration posed a "genuine threat" to "our general democracy, our peace and stability".
"It will actually be good for our democracy... (to) elect a new party to take over," said Khama.
Meanwhile, a red-clad Masisi addressed more than 1,000 supporters in his natal Moshupa village, 65 kilometres east of Gaborone.
"I have not packed any books in my office, I'm not about to leave the house I'm living in," Masisi told reporters beofre voting day.
"But if the unexpected should happen and we do not succeed, I will pack all my belongings peacefully and come home," he added.
"The rule of law remains supreme in Botswana," said Masisi. "And I'm addicted to it."
After the rally, supporter Boitumelo Dintwe told AFP she was confident Masisi would win.
"Masisi is intelligent, he is a real leader with a vision," said Dintwe, 39, who sells perfume.
"That one doesn't think," she added, referring to Khama.
The BDP is expected to garner around 44 percent of the vote, according to a pre-poll survey carried out by Afrobarometer.
"These elections are actually a test to how stable the country is," said University of Botswana politics lecturer Gladys Mokhahwa, .
"A test of... how entrenched democratic principles and mythos are in this country," she told AFP.
Polls opened around 6:30 am (0430 GMT) and are scheduled to close at 7:00 pm (1700 GMT).
Results are expected to be announced by the end of the week.