Back then I was taking an express bus to work every day, and we were stopped and backlogged on the overpass leading into the Midtown Tunnel (I don't live in Manhattan). One of the passengers, who happened to be standing yelled OMG a plane just smashed into the WTC (I was seated on the side with a clear view south from there all the way downtown), and everyone on my side of the bus looked out the windows to see it. From there it looked like it could've been an accident, but when we tried to call our offices, none of us had cell service, because the equipment was on the roof of one of the Towers (I don't recall which one).
Once we got through the Tunnel and took the highway downtown, we couldn't get all the way down, so several of us got off and walked the rest of the way, and the bus went home. What I saw when I turned the corner where the Towers were in full view, the second plane had crashed into the other Tower, and one of the regulars on the bus, with whom I was walking, and I stopped dead in our tracks as we were in total shock. That's when we knew it wasn't an accident.
When we got to the street we'd have to walk to our offices, everyone was walking north to get out of the area. I probably should've stayed on the bus and gone home, but large law firms here in NY (don't know about other cities), are open 24/7, so I was expected to be there. In fact, I was amazed at how many people actually made it into the office that day.
Anyway, the law firm I worked at was located in a building 4 blocks due east of the Towers. Now in the downtown area, most of the crosstown streets are very long, probably equal to a 1 1/2 to 2 standard block lengths, so maybe you could say 6-8 blocks, right in the line of sight of the Towers if we were on the west side of the building.
My office was on the west side of the building, and I heard, felt and saw the first Tower come down, and then I was out in the hallway yelling for everyone to close their windows, because a huge (I was on the 18th floor, and it was higher than my floor) cloud of grey smoke was heading our way.
By the time the second Tower came down, many of us were down in the law library watching everything on TV, and this time we were on the east side of the building and saw a whole lot of rubble falling after it passed over the roof of our building (40 stories high). The worst part for us was we had one client who had offices in the Towers, and one of our partner's wife was due to be at the Towers for a meeting. Luckily her meeting was postponed, so she wound up at our offices instead. A huge sigh of relief went up when she walked into the library where her husband was watching the action with the rest of us. One of the secretaries lost her brother-in-law that day, as he was one of the firefighters who had responded.
We were closed for over a week while they got the building back to a habitable condition, and most of us in my firm, myself included, walked around in the downtown area with surgical masks over our faces for months, because it took a long time to get the air quality back to anywhere near normal.
It also took a long time to get the express bus routes back to normal, because on a good day we drove down the street that passed the back of my building, and then turned around at the southern tip, and then drove up the street in front of the Towers, so needless to say that wasn't going to happen for a long time.
OK, that's a pretty long accounting, and there's lots of details I didn't include, but it was an experience I would NEVER wish on my worst enemy, that's how horrible a day it was, and a horrible week rewatching the events on TV for the week we were home before it was safe again for us to be in our office building.