Twiggy Garcia demanded that Blair accompany him to a police station as he was a "war criminal" who had launched an "unprovoked war against Iraq".
"I went over to him, put my hand on his shoulder and said: 'Mr Blair, this is a citizen's arrest for a crime against peace, namely your decision to launch an unprovoked war against Iraq. I am inviting you to accompany me to a police station to answer the charge'," Garcia told 'Vice' magazine.
"I think you should be more concerned about Syria," Blair said.
One of Blair's sons then got up and went to get the family's security team from downstairs, prompting Garcia to make a hurried exit.
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The UK's Serious and Organised Crime and Police Act 2005 gives citizens the right to detain a person whom they believe has broken the law.
He added: "I feel great. Lots of people have been contacting me to say well done. I'm still in disbelief that I got the opportunity to citizen's arrest the former prime minister.
"It all panned out pretty much how I thought it would, except that I didn't expect him to start debating with me."
A spokesperson for Blair said: "There is nothing to report here apart from fact that Mr Blair did offer to discuss the issue - that offer was declined and the individual walked off.
A Tramshed spokesperson declined to comment on the incident.