Irving, 67, said on Monday he now acknowledged that the Nazis systematically slaughtered Jews during World War II.
"History is like a constantly changing tree," he said as an eight-member jury and a panel of three judges prepared to hear charges that could put him behind bars for up to 10 years.
Irving arrived handcuffed at the court, carrying a copy of one of his most controversial books, Hitler's War, which challenges the extent of the Holocaust.
"I made a mistake when I said there were no gas chambers at Auschwitz," Irving told the court, speaking in German.
But he said he had never written a book about the Holocaust, which he called "just a fragment of my area of interest".
"In no way did I deny the killings of millions of people by the Nazis," Irving testified.
The historian has been in custody since his arrest in November on charges stemming from two speeches he gave in Austria in 1989 in which he was accused of denying the Nazis' extermination of six million Jews.
A verdict could come later on Monday.
Irving's trial comes against a background of a new and fierce debate over freedom of expression in Europe, where the printing and reprinting of cartoons of Prophet Muhammad, deemed blasphemous, has triggered violent protests worldwide.
Irving had tried to win his provisional release on $24,000 bail, but a Vienna court refused, saying it considered him a flight risk.
The trial comes as protests rage
over controversial cartoons
His lawyer, Elmar Kresbach, said last month the controversial Third Reich historian was getting up to 300 pieces of fan mail a week from supporters around the world.
He also said that while the historian was in detention, he was writing his memoirs under the working title Irving's War.
Irving was arrested on 11 November in the southern Austrian province of Styria on a warrant issued in 1989.
He was charged under a federal law that makes it a crime to publicly diminish, deny or justify the Holocaust.
Within two weeks of his arrest, Irving asserted through his lawyer that he had come to acknowledge the existence of Nazi-era gas chambers.
In the past, however, Irving had said that Adolf Hitler knew little if anything about the Holocaust, and had been quoted as saying there was "not one shred of evidence" the Nazis carried out their Final Solution to exterminate the Jewish population on such a massive scale.
Vienna's national court, where the trial is being held, ordered the balcony gallery to be closed to prevent projectiles from being thrown at the bench, the newspaper Die Presse reported on Sunday.
Irving earlier said that there was
no evidence of the Holocaust
It quoted officials as saying they were bracing for Irving's supporters to give him the Nazi salute or shout out pro-Hitler slogans during the trial, which will continue into Tuesday if a verdict is not reached on Monday.
Irving is the author of nearly 30 books, including Hitler's War, which challenges the extent of the Holocaust, and has contended most of those who died at concentration camps such as Auschwitz succumbed to diseases such as typhus rather than execution.
In 2000, Irving sued American Holocaust scholar Deborah Lipstadt for defamation in a British court, but lost.
The presiding judge in that case, Charles Gray, wrote that Irving was "an active Holocaust denier ... anti-Semitic and racist".
Irving has had numerous run-ins with the law over the years.
In 1992, a judge in Germany fined him the equivalent of $6000 for publicly saying the Nazi gas chambers at Auschwitz were a hoax.