can I photograph this person legally without their permission?
You can photograph this person, you cannot publish this photo in any form (including websites) unless you have the permission from that person or you have paid them.
That is true for all photos that show a person in such a way that he/she can be identified (and that means not only face but for example also the items that may point directly to a specific person).
There are two exceptions:
- photos of public figures / celebrities while performing their public / professional functions
- documentary shots of scenes (including urban ones), landscapes or gatherings. The rule of thumb is that it's legal if you can remove the person from the image and it will not affect the subject and / or character of the photo.
The above are the legal regulations... the reality is usually completely different.