If found guilty Mr de Villepin faces up to five years in prison
Lawyers for the former French Prime Minister, Dominique de Villepin, say they will sue President Nicolas Sarkozy for having called him "guilty".
Mr Sarkozy is accused of violating the principle of presumption of innocence.
Mr de Villepin, an arch-rival of Mr Sarkozy, went on trial on Monday over the so-called "Clearstream" affair.
It is alleged that Mr de Villepin tried to manipulate a judicial investigation to hurt Mr Sarkozy's chances of winning the 2007 presidential election.
Asked for a comment on the trial, Mr Sarkozy told French television on Wednesday: "After a two-year investigation, two independent investigating judges ruled that the guilty parties should be tried before a criminal court."
Correspondents say that, as president, Mr Sarkozy enjoys immunity from prosecution, but that the defence team may use the "guilty" comment to bolster an appeal on behalf of Mr de Villepin. They could argue that Mr Sarkozy is using political influence to pervert the course of justice.
"This is a scandalous violation of fundamental principles," said defence lawyer Henri Leclerc.
Sarkozy a plaintiff
Controversially, Mr Sarkozy - himself a lawyer by training - is a civil plaintiff in the case. As president he will not have to testify, but he is represented by lawyers in court.
Mr de Villepin stands accused of "complicity in false accusation, complicity in using forgeries, receipt of stolen property and breach of trust" and is expected to testify next week.
THE CLEARSTREAM ACCUSED
Dominique de Villepin: Former PM, 55, denies accusations he tried to circulate details of Sarkozy's illegal bank accounts, even though he knew they were fake
Jean-Louis Gergorin: Former Airbus VP, 63, accused of faking Clearstream bank accounts as part of Airbus power struggle, says he accepted accounts in good faith
Imad Lahoud: Computer expert, 42, says he faked Clearstream accounts on Gergorin's behalf, introducing Sarkozy's name on Villepin's instructions
Florian Bourges: Accountant, 31, accused of stealing Clearstream documents and breach of trust
Denis Robert: Journalist and author, 41, who broke the story, accused of dealing in stolen property and breach of trust
The case dates back five years, when Nicolas Sarkozy's name appeared on a list of top politicians and businessmen sent to Mr de Villepin.
It was alleged those named on the list had received bribes from international arms sales.
But when a judge concluded that the list of names was bogus, the inquiry switched to who was behind the spurious allegations.
Mr de Villepin and four others are accused of plotting against Mr Sarkozy to spoil his chances of winning the presidential election.
The name Clearstream comes from the Luxembourg bank where the individuals on the list - including Mr Sarkozy - were claimed to be account holders.
If found guilty, Mr de Villepin could face up to five years in jail and a 45,000 euro (£41,000) fine.
The former prime minister denies any wrongdoing.
"I am here because of one man's will. I am here because of the dogged determination of one man, Nicolas Sarkozy, who is also president of the French republic," he told journalists on Monday.
"I will come out of this a free man and exonerated," he said.
Witnesses are due to include another former prime minister, Jean-Pierre Raffarin, and some past and present intelligence chiefs.
Mr de Villepin and Mr Sarkozy were both ministers under President Jacques Chirac, but were intense rivals.
Mr de Villepin was preferred by Mr Chirac, but it was Mr Sarkozy who won party support to succeed the president.