Where did the people come from?

We see the soldiers and police going door to door evacuating people from their homes near the river and then see several scenes of deserted streets. But as soon as Behemoth comes out of the river, it's like someone kicked a fire ant hill and people go running from everywhere. And they just keep coming and coming as the monster moves thru the streets. I also liked the police having to hold back crowds when the monster appears, my A$$ would have been long gone with no need to look.