In articles like this:
Croatia says 'forcing' Hungary to take in migrants
Over and over I'm seeing numbers like 5,000, 10,000, 20,000. Those are small town sized populations. While it is certainly a good chunk of people, it's really not that much. I've been to concerts with 200,000. I go to a festival every year with 80,000 and it's in a park. The racetrack here holds close to 100,000. The college football stadium here holds 100,000 seats.
It would certainly not be ideal and it would not work for an extended amount of time, but don't you have stadiums or large parks or something of that nature where the people can stay as a temporary solution?