In a statement to the High Court in Kuala Lumpur, Mr Anwar denied accusations that he had had sexual relations with a former male aide.
The 64-year-old called the allegations "a vile and desperate attempt at character assassination".
Mr Anwar faces up to 20 years in jail if convicted.
In defiant mood, Mr Anwar read for more than an hour from a prepared text, as the defence began presenting its case in the often-delayed trial.
The charge that he sodomised Mohamad Saiful Bukhari Azlan was "a blatant and vicious lie" orchestrated by his political enemies, he told the court.
"This entire process is nothing but a conspiracy by Prime Minister Najib Razak to send me into political oblivion by attempting once again to put me behind bars," he said.'Charade'
Mr Anwar was charged shortly after he led the opposition coalition to significant gains in the 2008 election.
His lawyers will call witnesses over the next few weeks, including the prime minister. The government has denied any conspiracy.
Mr Anwar's legal team wants to prove that this sodomy trial is a "charade " to push him out of politics, says the BBC's Jennifer Pak, who was at the court.
But that will not be easy, she adds.
The judge has already stated that the testimony by Mr Anwar's accuser and key DNA evidence linking him to the crime are both "credible" and "reliable".
Mr Anwar says this is a pre-judgement of his case and accuses the judge of denying him a fair trial.
That is why he has refused to testify on the stand, choosing instead to read out a statement so that he cannot be cross-examined by the prosecution, our correspondent says.
Mr Anwar was imprisoned on separate sex and corruption charges in 1998, and freed on appeal in 2004.
Sodomy is illegal in Malaysia, even among consenting adults.