Do you have an assignment letter or letter from a book publisher? What is your purpose for the photographs because if you can not answer the questions I have asked you, then why do you want to take these photographs?
It's 2013 sir, too many hacks out there calling them selves "Street" photographers and slathering pictures of people they do not know or care about all over sites like Flickr like they were human prey who's heads are on display in a taxidermy shop. The days of Bresson and Winogrand have been pounded deeply into a permanent grave thanks to amateur camera owners who think of no one but them selves. Heck, I shoot professionally and do not bother people I am not assigned to photograph, I think it is super creepy in the internet age.
I'm afraid that unless you have a professional reason with official backing to do this, you are out of luck and rightfully so, no one wants to be the subject of photos they have no idea of what they are to become or why they are being taken.
Get a magazine or book assignment or, well....leave these people alone.