Wulff, the only German former head of state to face charges in court, voiced indignation at the claims against him, demanded "justice" and said that "the personal damage that I and my family have suffered will stay, probably a lifetime."
Wearing the Federal Cross of Merit on his lapel, Wulff, 54, confidently vowed to restore his honour after he was forced to step down in disgrace in February 2012 amid personal turmoil that also saw him separate from his wife within a year.
Speaking earlier outside court, he signalled hopes for a return to public life after his months of relative seclusion, saying he would like to dedicate himself again "to the issues that have always been close to my heart."
Wulff faces a charge of accepting favours while in office that, compared to political scandals elsewhere, may seem trifling. Many commentators, however, say the trial proves that no public official stands above the law.
If found guilty, the former largely ceremonial head of state theoretically faces up to three years in jail or a fine.
The one-time conservative rising star is accused of having allowed a film producer friend to pay some of his travel expenses during a 2008 Munich visit while he was state premier of Lower Saxony, in return for helping him promote a movie project.
Prosecutors charge that the film industry financier, David Groenewold, paid part of Wulff's hotel room bill, babysitting costs and a restaurant meal, totalling just over €700 euros.
Wulff 'already punished enough'
The accusation is that Wulff became guilty of influence-peddling, possibly corruption, when he later lobbied the then CEO of industrial conglomerate Siemens, Peter Loescher, for support for a Groenewold movie project.
The ex-president said a letter was indeed sent from his premier's office at the time, but he described this as normal, adding that it was penned by a state secretary and that "not one line was from me."
He told the court he was motivated not by his friendship with co-accused Groenewold, 40, but because he found the World War II era story linked to the German company compelling.
The 2009 movie in question, "John Rabe", is based on the true story of a Siemens manager who saved more than 200,000 Chinese when Japanese forces committed the Nanjing massacre in 1937-38.
The allegation is the only remaining charge prosecutors are levelling against Wulff after initially investigating other claims he took favours from wealthy friends, including luxury holidays and a cheap home loan.
Opinion is split on the case, with some commentators questioning whether the alleged crime justifies a scheduled 22-day trial with 46 witnesses set to last until April 2014.
The editor of the mass-circulation daily Bild, whose coverage first broke the wider scandal, Thursday asked whether a state premier could really be bought for such a sum and argued that Wulff "has already been punished enough."
The commentary also asked whether state prosecutors were being "petty and stubborn" in seeking to prove wrong-doing at all cost to justify in hindsight the launch of the criminal probe that sparked Wulff's downfall.
The formal accusations against Wulff and Groenewold were reduced from the more serious corruption and bribery charges that carry up to five years' jail.
The ex-president has opted to fight the case rather than settle it with a €20,000 fine. Judge Frank Rosenow ended proceedings after about three hours. The trial is set to continue next Thursday when the bench will call the first four witnesses.