Protecting Tiananmen Square

I was walking back to the hotel tonight and my route happened to pass by Tiananmen Square. I found it curious to see that all entries to the square were blocked at night and were protected by a soldier at each entrance ... so no entry is possible. It's quite a massive operation to do something like that given that it's the largest square in the world!

I suppose that's one way of keeping it clean and prevent vandalism ... can't really think of any other reasons why they do it, but I will try to find out from some people here.