Details of the confrontation emerged as a public inquiry into the atrocity began in Manchester.
The spectator, who can only be named as Witness A, said he approached a man resembling Salman Abedi because he looked “out of place” on the night of the attack of May 22, 2017.
Witness A testified that the suspect man was carrying a large backpack in the crowded room.
Paul Greaney QC, lawyer for the investigation, said at the opening hearing: “(The witness) asked the man what you had in your backpack, but he got no answer. .
“(The witness) then said, ‘That doesn’t look very good, you know, you with a bag in a place like this. What are you doing? ”
The man reportedly replied, “I’m expecting someone, buddy. Do you have time? What time is it? ”
Witness A testified that he was “fobbed” by security guard Mohammed Agha when he warned him about the suspect, according to the investigation.
The witness spoke to Mr. Agha at 10:14 p.m., approximately 17 minutes before the explosion.
Mr. Agha then spoke to his colleague Kyle Lawler about the matter eight minutes before the bomb went off.
But neither security screening nor anyone else was made aware of the suspicious activity, the hearing said.
Mr. Lawler allegedly attempted to radio his security colleagues, but was unable to get through.
He then spotted the man getting up and heading towards the entrance to the arena.
Mr Lawler said in a statement: “I just froze and didn’t broadcast anything on the radio. I knew then that it was too late. ”
Mr Greaney said the investigation would examine whether any errors had been made: “Why didn’t Mohammed Agha and Kyle Lawler notify the control room or anyone else between 10:14 p.m. and 10 p.m. 31 of Witness A’s report of a suspicious man, with backpack, on the mezzanine level of the town hall?
“If their failure to do so was culpable, was it the result of inadequate training and / or instruction or, on the contrary, the consequence of individual error of ineptitude? ”
Another man concerned about Abedi reported him to a police officer.
Mr Greaney added: “Whether it is missed opportunities to prevent what happened that night, to reduce its magnitude… (is) a matter of very considerable importance. “
The investigation also learned that fire and ambulance teams have been delayed in entering the home – an issue that experts consider “very worrying”.
Survivor Sean Gardner told Sky News that he and many other people in the household felt abandoned for at least 25 minutes after the explosion: “It obviously left very deep scars.
“There would always have been a five or 10 minute gap, but it was 25 minutes before we had any help.
“We need to hear the reasons for the delays and hope these are the right decisions. ”
An earlier report by Lord Kerslake found that the Greater Manchester Fire and Rescue Service had played “no significant role” for almost two hours and that there was “little awareness of what was going on in the arena ”.
The investigation also learned that Salman Abedi’s older brother, Ismail, had received a text message from his family in Libya three and a half hours before the attack, offering him God’s blessing, according to the investigation.
He refused to participate in the investigation in case he incriminated himself.
The investigation is expected to continue until spring 2021.