Originally Posted by chtiet
Yes, but they cannot detain you - they can only refuse you entry to the US. After all, it's Canadian soil, and you're protected by Canadian law (assuming you're in Canada legally).
Actually, read the act. The area where customs is located is the same as US soil. Its treated as a part of the embassy. They work with Canadian law enforcement if you act odd, or break any rules while waiting to enter the US. They can't detain you, but they can turn you over to Canadian authorities, who can then detain you. Yes, you can turn away if you don't want to answer their questions, but the next time you try to enter, there will be a big note on your file and you'll probably be answering more questions than before
For the exact law, check
http://laws.justice.gc.ca/en/p-19.3/93336.html