Alex Salmond is being questioned by MSPs over his claims of a "malicious and concerted" conspiracy against him.
The former first minister is appearing at Holyrood as part of an inquiry into the government's mishandling of sexual harassment complaints about him.
The four-hour session is expected to focus on that botched probe and the accusations made by Mr Salmond.
Nicola Sturgeon has challenged her predecessor to prove his claims, saying there is "not a shred of evidence".
The first minister is expected to give evidence to the inquiry committee herself on Wednesday.
The Holyrood session with Mr Salmond began at 12:30.
It has been delayed on several occasions due to a row over his written submissions, which make a range of accusations about the conduct of the first minister, government officials, the SNP and the prosecution service.
He wrote that there had been "a deliberate, prolonged, malicious and concerted effort among a range of individuals within the Scottish government and the SNP to damage my reputation, even to the extent of having me imprisoned".
And he claimed there had been a "complete breakdown of the necessary barriers which should exist between government, political party and indeed the prosecution authorities".
He has also taken aim directly at Ms Sturgeon, accusing her of "repeatedly" misleading parliament about her involvement in the row and of breaching the ministerial code.
The SNP leader has hit back Mr Salmond's claims, saying he is "creating an alternative reality in which the organs of the state...are all part of some wild conspiracy against him for reasons I can't explain".
And she has challenged him to back the allegations up with proof, saying he has an "obligation" to "replace the insinuation and assertion we have heard over several months now with evidence".
It's been more than six years since Alex Salmond stood down as first minister after losing the independence referendum and now he's back at Holyrood, with what looks like a vengeance.
He'll accuse his successor, Nicola Sturgeon, of repeatedly breaking the rules for ministers in her handling of harassment complaints against him.
He'll point the finger at those in the Scottish government and the SNP - both of which he used to lead - that he believes plotted against him.
He'll allege that Scotland's prosecution service has assisted them by improperly withholding information that could back up his conspiracy claims.
This is his opportunity to convince MSPs his allegations have substance and are not the "alternative reality" Ms Sturgeon thinks he has created.
Instead of commanding the main chamber as he did as first minister, today Alex Salmond will appear as a witness before a parliamentary committee.
Given the enormity of his allegations, his four-hour evidence session will be one of the most notable contributions in Holyrood's short history.
The Scottish Parliament inquiry was set up after the government admitted its investigation of two harassment complaints against Mr Salmond had been unlawful.
An internal probe had upheld the complaints - which dated back to when he was first minister - but the decision report was torn up after the government conceded in court that the investigation had been "procedurally unfair" and "tainted by apparent bias".
Mr Salmond was awarded more than ??500,000 in legal expenses following the judicial review case, and was subsequently acquitted of sexual assault charges in a separate High Court trial.
The inquiry session will see Mr Salmond give an opening statement, and then take questions from the MSPs on the committee for up to four hours.
The group consists of four SNP MSPs - one of whom, Deputy Presiding Officer Linda Fabiani, acts as convener - as well as two from the Scottish Conservatives, one from each of Scottish Labour and the Lib Dems, and independent MSP Andy Wightman, formerly of the Greens.
The committee has frequently been at odds with the key players in the row, having complained of "obstruction" and submissions which provide "very few clear answers".
The Podlitical team break down the latest developments in the Alex Salmond saga as his row with Nicola Sturgeon escalates, and look ahead to his long-awaited appearance at the inquiry.
You can listen to the latest episode here or wherever you get your podcasts.
There has also been a lengthy row over whether one of Mr Salmond's submissions - which accuses Ms Sturgeon of breaching the ministerial code - could be published at all.
Members twice voted against publishing it at all due to legal issues, before Holyrood's cross-party management board intervened to say it could be. However, the Crown Office then raised "grave concerns" about its contents, and the paper was taken off the parliament's website and redacted - forcing yet another delay to Mr Salmond's long-awaited session.
The committee has agreed to apply to the High Court for "specific guidance" about what can and cannot be published, and to attempt to requisition further documents linked to Mr Salmond's criminal trial from the Crown Office.
Ms Sturgeon is due to give evidence on Wednesday, 3 March - which she said was the sixth date she had now had in her diary to face the committee.