Radical preacher Abu Hamza al-Masri was found guilty because a fair trial was "impossible", a court has heard.
The cleric has been vilified, say his lawyers
His lawyers told the Appeal Court his trial was prejudiced by "unique" global events and a media hate-campaign.
Abu Hamza, jailed for seven years in February, is appealing against his conviction for soliciting to murder and inciting racial hatred.
During the trial, prosecutors said Abu Hamza used a north London mosque to recruit people into Islamist extremism.
The Egyptian-born cleric, who is being held at Belmarsh high-security prison, was jailed after being found guilty of 11 out of 15 charges.
The charges related to what prosecutors said were a series of inflammatory speeches and recordings he made as a preacher at Finsbury Park mosque and other locations.
ABU HAMZA VERDICTS, FEB 2006
Guilty of 6 charges of soliciting to murder
Guilty of 3 charges related to "stirring up racial hatred"
Guilty of 1 charge of owning recordings related to "stirring up racial hatred"
Guilty of 1 charge of possessing "terrorist encyclopaedia"
Not guilty of 3 charges of soliciting to murder
Not guilty of 1 charge related to "stirring up racial hatred"
Edward Fitzgerald QC, representing 48-year-old Abu Hamza, told the Court of Appeal it had been "unfair and oppressive" to launch a prosecution in 2004 over speeches made between 1997 and 2000.
Police had taken no action at the time, even though officers had taken away the recordings for analysis.
The prosecution came amid a "relentless campaign of adverse media publicity condemning him as a preacher of hate and inciter of violence", said Mr Fitzgerald, some of it driven by the public battle led by the Home Secretary to strip Abu Hamza of his citizenship.
This campaign, argued Mr Fitzgerald, combined with the unique events of the 9/11 attacks and subsequent 7 July 2005 London bombings had prejudiced the trial.
"Inadmissible evidence relating to his alleged involvement in actual crimes of terrorism was relentlessly highlighted and referred to in public," said Mr Fitzgerald.
"Because of the delay in prosecuting the appellant, the atmosphere had been poisoned against him in a way that would not have happened had he been subjected to a timely prosecution."
Meetings with MI5
During his month-long trial, Abu Hamza told the Old Bailey the speeches he was being prosecuted over had been well known to the authorities, including MI5.
Special Branch officers had seized the tapes from his home and he had held a series of discussions with the authorities about his views and intentions.
The preacher said he had left those meetings believing he would not face prosecution, partly because police officers returned his recordings and other documents.
However, prosecutors successfully argued he had used Finsbury Park mosque as a secure base from which he could act as a recruitment sergeant for global terrorism.
They said he was preaching racial-hatred against Jews and other non-Muslims and had owned a multi-volume "terrorism manual" known as the Encyclopaedia of Afghan Jihad. This encyclopaedia, said Abu Hamza, was one of the documents that police gave back to him.
The two-day appeal is being heard by the Lord Chief Justice, Lord Phillips, sitting with Mr Justice Penry-Davey and Mr Justice Pitchford, at London's Court of Appeal.
Abu Hamza also faces a continuing battle against extradition to the United States to face allegations of trying to establish a terrorist training camp.
The extradition attempt has been postponed until the outcome of the appeal, which continues on Tuesday.