The man at the centre of the alleged plot to told a court on Tuesday that there were "50 mistakes" in the indictment against him.
Rafik Mohamad Yousef was speaking as he and two other men went on trial accused of being members of a terrorist group and conspiring to kill Allawi in Berlin in December 2004.
Prosecutors say the three are members of Ansar al-Islam, an Iraqi group which the US has linked to al-Qaeda. One of them, Ata Abdoulaziz Rashid, is described as the organisation's senior leader in Germany, where intelligence officials estimate the group has about 100 supporters.
Prosecutors allege that he and the third defendant, Mazen Ali Hussein, transferred about 75,000 euros ($94,000) to Iraq to finance attacks there.
The three men - referred to in court as Mr Rafik, Mr Ata and Mr Mazen - were arrested on the basis of intercepted phone calls in which Rafik was alleged to have sought the go-ahead to kill Allawi.
When the others agreed, he drove through central Berlin to check out a Deutsche Bank building where the Iraqi leader was due to hold a meeting the next day, according to the charges. Police arrested all three that night.
Investigators said at the time they had not found weapons or explosives, and prosecutors have not yet said how they believe the attack was to have been carried out.
The hearing, in a Stuttgart courtroom specially built in the 1970s, began with technical wrangling as lawyers for Ata and Rafik demanded that the charge sheet, shown to them in Kurdish, be translated into Arabic instead.
Rafik then told the court in German: "There are 50 mistakes, and I can prove it."
The presiding judge turned down the language demand, on the grounds that both men had asked for translation into Kurdish when applying for residence in Germany in the 1990s.
In a separate case, two other Iraqi men went on trial in Munich on Tuesday accused of providing logistical and financial support for Ansar al-Islam.
The cases are part of a series of investigations into alleged links between European-based Islamists and fighters trying to bring down the Baghdad government and drive US-led forces out of Iraq.
An Ansar al-Islam member was jailed for seven years by a Munich court in January for recruiting fighters and raising cash for those fighting the US presence in Iraq. Another Iraqi man accused of funding the group was arrested at Frankfurt airport last week.