As other people has mentioned here, always ask for permition before taking a picture of any persons. One thing to remember is to be careful when taking pictures in public places to.
I was arrested in Dar es Salaam, Tanzania, for taking pictures.
The story was me, sitting in the back seat of a car, taking photos on the main road in to Dar es Salaam. Unfortunately a police truck was passing us and someone spotted my camera. Even thoug I was not taking pictures at that moment they stopped the car, pulled me out, and took me to the police station in Dar es Salaam. There I had to stay for one hour beeing questioned by the police captain. I was free to go after somewhere around one hour, but had to delete ALL the pictures on my camera. I was told it was not legal to take any pictures of roads, buildings, offical vehicles, and so on. It was my duty to know the law of Tanzania when entering the country... What could I say? He was right, I was wrong.
In a situation like that it's always vice wo be very sorry, regret what you have done and delete your pictures with out any fuzz, never argue.