The former prime minister was reportedly approached by Twiggy Garcia while he was dining with family and friends at the Tramshed restaurant in Shoreditch.
Mr Garcia said he was making a citizen’s arrest on the grounds that Mr Blair was a “war criminal” who had launched an “unprovoked war against Iraq”.
The barman told the Vice magazine website: “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’.
“He said, ‘No, shouldn’t you be worried about Syria?’
“Then he asked me, ‘But don’t you agree that Saddam was a brutal dictator and he needed to be removed?’ and I replied, ‘Not by an illegal war’.”
Mr Garcia said Mr Blair’s companions stepped in after he again asked the politician to accompany him to a police station.
“One of his sons got up and went to get the plain clothes security [staff] from downstairs. I decided to get out of there.”
Mr Garcia, who is also an aspiring music producer, said he had resigned from his job immediately.
A spokesman for Mr Blair said: “Mr Blair did offer to discuss the issue. That offer was declined and the individual walked off.
“Nothing else happened. Everyone is fine and they had a great time at the restaurant.”
A spokeswoman for Tramshed declined to comment.