South Africa's parliament is due to vote on an opposition-sponsored motion to impeach President Jacob Zuma.
The Democratic Alliance (DA) said he was no longer fit to govern after the country's highest court ruled last week that he had breached the constitution by failing to repay public money used to upgrade his private residence.
The governing African National Congress (ANC) is expected to defeat the motion. It denounced the impeachment proceedings as a publicity stunt.
The DA said it would demand a secret ballot in the hope that ANC backbenchers would defy the party by helping it obtain the two-thirds majority - 267 MPs out of 400 - required in the lower house, the National Assembly, to impeach Mr Zuma.
However, speaker Baleka Mbete has rejected the proposal, saying it is not allowed in terms of parliamentary rules, local media reports say.
The DA has 89 seats in parliament and all opposition parties combined 151.
Mr Zuma is the first president to face an impeachment vote since minority rule ended in 1994.
He has been dogged by allegations of corruption since before he was elected president in 2009.
He was accused of taking bribes over an arms deal, but he denied the allegation and the charges were controversially dropped just before he took office.
He later found himself at the centre of controversy over the use of $23m (£15m) of public money to upgrade his private home in the rural area of Nkandla in KwaZulu-Natal province.
In 2014, South Africa's anti-corruption watchdog Thuli Madonsela ordered him to repay a portion of the money.
Mr Zuma had "unduly benefited" from the building of a swimming pool, amphitheatre, cattle enclosure and chicken run, Ms Madonsela said.
The DA and left-wing Economic Freedom Fighters (EFF) party went to the Constitutional Court to challenge Mr Zuma's refusal to pay the money.
It ruled against Mr Zuma, and said the law was the "sharp and mighty sword that stands ready to chop the ugly head of impunity from its stiffened neck".
The court also condemned parliament for failing to hold Mr Zuma accountable, and rejecting the public protector's findings.
In a televised address to the nation on Friday, Mr Zuma apologised for the "frustration" caused by the long-running controversy and said he would abide by the ruling.
He had acted "in good faith" and "never knowingly and deliberately set out to violate the constitution", he added.
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