"I have never been a member of Real IRA or helped them any way,” Campbell, 40, said in his final speech before the court.
He maintained that the case is "built on illegal activity of MI5," the British secret service, adding that he is prosecuted due to his brother Liam who is titled as one of the leaders of the Real IRA.
“I am guilty because my brother has a record. How can this be right? If my brother was a priest, does that make me a priest?” Michael Campbell said in his address to the court.
He also stated that the case may have contained forged audio recordings.
The Lithuanian Court of Appeals is to hand down its ruling on October 2.
The court had earlier decided to interview Liam Campbell and another suspected Irishman Brendan McGuigan in the case but said it didn't receive a reply from British and Irish authorities.
The United Kingdom has refused to extradite Liam Campbell to Lithuania due to what it says are poor prison conditions in the Baltic state.
The Friday's hearing was attended by four Irish parliamentarians from Dublin. They said they came to Vilnius to make sure that human rights are observed.
MP Martin Ferris from the Sinn Fein party, which is the political wing of the IRA, said that the legislators came to Vilnius at the request of the convict's family.
“While we respect the rights of the Lithuanian judicial system to deal with the case, we are here strictly on humanitarian grounds in relation to the conditions that he has been held under," Ferris told journalists after the hearing.
He has also emphasized that Ireland wants peaceful solutions.
Lithuanian prosecutors have asked the court to extend his sentence from 12 to 16 years in prison.
Michael Campbell was found guilty of several different crimes, namely, support to a terrorist group, unlawful arms possession, and attempted smuggling.
Michael Campbell was arrested in Vilnius in January 2008 after purchasing arms from an undercover agent.
Lithuanian prosecutors have cited classified witnesses as saying he inquired about the amount of explosives needed to blow up a governmental automobile and said he would use the arms against humans.
In prosecutors' words, British agent Robert Jardine said the Real IRA earmarked 100,000 euros for arms purchases.
The Real IRA is a paramilitary group that opposes British rule of Northern Ireland. The group is seen as a terrorist organization by the Council of Europe.
