It was barely early afternoon when the dome appeared over Manehattan. Most didn't see it happen, some didn't even notice it for a while. Some would have only noticed it hours later if at all, had they not been called out or to a window to witness it.
It did not make a show, of any kind, when it appeared. One moment it wasn't there, the next it was. Translucent, largely not blocking the light, barely visible at some angles and not too much more noticeable at others. The first few who spotted it, mostly those already close to the edge of the city, approached it with a mix of caution, interest and confusion. First they realised it did not hurt to touch it, and they were relieved. Then the realisation that they were trapped there set in.
Soon ponies were gathered in crowds around town, looking up at the sky and wondering what was happening, talking and asking and occasionally sending a pegasus up to check, again, that it really was there, that it really was holding them in. Unicorns near the border soon began to try and fail to teleport outside, some ponies began to circle around the edge looking for an exit, some began to dig and found quickly that the barrier extended underground too, at least as far as they could reach in such a brief time.
No one really knew what was happening. Many tried to contact the outside in some way, none succeeded. When guards arrived at the edge of the barrier, it took minutes before they were finally noticed by someone inside. At first they tried to bang against the barrier, but sound didn't pass through it. That became apparent when they tried to speak with those who approached them, and when those ponies tried to speak back.
Communication through written means was fairly quickly arranged, while a few of the guards were sent to scout the outside of the barrier and look for any exit points. None would be found, and, knowing they did not have much time, by the time the Sun had disappeared the guards stopped searching. The ponies inside would keep at it for a while, and swore they would be back the next day, and no pony had anything particular against that. There was not much else they could be doing in there, after all.
The citizens were, following Twilight's orders, instructed not to worry. They were told the barrier would disappear in a week at most, to not stress over it, and to continue on their lives as normal wherever possible. At most, if they were to act differently because of the situation, the Princess had said to treat it like a vacation period. To enjoy themselves and relax.
Few of the guards had a proper idea of what was going to happen, though those that did were ready to give the rest the talk when the time approached. Twilight really hoped it wouldn't be a necessity.