Blair's appearance starts off an important week in the judge-led inquiry
into media ethics in the wake of a phone hacking scandal last year.
Lord Justice Brian Leveson is leading the inquiry and is examining whether politicians failed to curb the activities of renegade journalists because they were too close to the media.
Just before former Prime Minister Blair testified, a protester burst into the court room and shouted, "This man should be arrested for war crimes!" before being removed by security from the courtroom.
The man apparently gained entrance to the court through a secure corridor and Leveson was very apologetic to Blair about the situation.
Blair briefly addressed the man's accusations before continuing his testimony in the inquiry.