Update 4.30pm: South Africa's parliament has begun voting on a no-confidence motion in President Jacob Zuma amid numerous allegations of corruption.
While the president has survived several such votes in the past, this is the first to be conducted by secret ballot, and if it succeeds he will have to resign immediately.
The no-confidence motion needs 201 votes to succeed, or a simple majority of parliament's 400 seats, but because five parliament seats are vacant, the opposition wants the count to be based on 395 seats instead.
South Africa's parliament is preparing to vote on a motion of no confidence in embattled President Jacob Zuma that could force him to resign after months of growing anger over alleged corruption.
Mr Zuma has survived six previous no-confidence votes in parliament, but this is the first to be held by secret ballot after parliamentary speaker Baleka Mbete on Monday made the surprise decision to allow it.
Opposition parties hope it will encourage disgruntled legislators with the ruling African National Congress party to vote against Mr Zuma, who has faced numerous allegations of corruption while South Africa's economy has fallen into recession.
Widespread frustration over Mr Zuma has hurt the ANC, the former liberation movement that has led South Africa since the end of white minority rule and the first all-race elections in 1994.
On Tuesday, former president Thabo Mbeki said ANC legislators must ask themselves if they have confidence in Mr Zuma when they go to vote, according to a video posted by a Nairobi-based journalist on Twitter.
"Those MPs must recall that they are the representatives of the people, and must therefore represent the people in terms of what they do this afternoon," Mr Mbeki told reporters.
The ANC holds a majority of the 400 parliament seats, and the party has repeatedly said its members will not support the opposition-led attempt to unseat the president.
The party has 249 parliamentary seats, five of which are vacant, said a party spokeswoman, Nonceba Mhlauli.
The no-confidence motion needs 201 votes to succeed.
Demonstrations for and against Mr Zuma, who has led South Africa since 2009, were taking place in front of the parliament building in Cape Town before the much-anticipated vote.
If the motion succeeds, Mr Zuma and his cabinet must resign immediately and Ms Mbete will take over as acting president, according to Pierre de Vos, a constitutional expert and law professor at the University of Cape Town.