The ex-Beatle spoke at the Television Critics Association press tour via satellite from Ohio on Thursday to promote his new documentary The Love We Make, which chronicles McCartney's efforts to put together the October 2001 'Concert for New York City'.
McCartney recalled being on the runway at New York's JFK International Airport when he first saw the burning World Trade Centre towers on the morning of September 11, 2001.
"Out of the window, on the right-hand side of the airplane, you could see the Twin Towers. First of all you could see one plume of smoke and you could see two shortly thereafter and I said, 'Well, that's an optical illusion… or it's probably just some little fire or something, but it does look pretty serious'," the singer remembered.
He continued: "We were just looking at that for a while and then suddenly one of the stewards… came to me and said, 'There's something really serious [that has] happened in New York and we've got to get you out of here'.
"I ended up in Long Island watching it on TV… wanting to go into New York, but nobody was allowed back in, and while I was sitting out there twiddling my thumbs thinking what to do, 'Was there any role I could play in this?' The idea came to me that maybe we could do a concert, get something together."
McCartney eventually enlisted documentary filmmaker Albert Maysles to help him capture the mood of New York City in the days following the attack.
"There was fear in the air and I'd never experienced that, particularly in New York, so this is where the idea for doing the ['Concert for New York City'] came about," the star said.
McCartney went on to say that The Love We Make ultimately stands as his love letter to New York City.
The musician explained: "When you talk to me about New York now, it's the people [I think of], because I married a New York girl - Linda - and I'm about to marry another one (Nancy Shevell), so I think I would think first of Linda and her family and our family and her connections with New York and then my upcoming connection.
"Then, I think after that… Ed Sullivan, Shea Stadium, [the] 9/11 concert and many great concerts there. Most recently Yankees, Citifield before that, [the] closing Shea with Billy Joel. So I have a lot of connections. I love New York."
The Love We Make airs September 10 on Showtime.
McCartney also revealed during the TCA press conference that he believes his phone was hacked and will be making an inquiry with police.
> Sir Paul McCartney to open London 2012 Olympics?
Watch Paul McCartney perform 'Freedom' and 'Let It Be' at 'The Concert for New York City' below: