As with any foriegn city you visit be careful of people trying to sell you rubbish. Also be aware of people trying to earn money off you. If you are in a car and a man helps you to park he will expect money! Dont be alarmed, there are far more dangerous places in the world than Porto
While I was in Porto I was followed several times by various strange men. They ranged from curious locals to men seeking a love interest to downright shady characters. I only really felt in imminent danger once. Even holding up my hand and saying stop and yelling Policia did not always deter them. I was traveling alone and I think it would have helped to have a male companion to act as a deterrent, but if you are female and traveling solo, then I recommend a few things. Get in a crowd if you notice someone is following you, do an about face and walk in the opposite direction to see if they continue, look at them and say No and Policia, or if need be hop a public bus or other form of transportation to get away from them (they are not likely to want to pay to follow you around). Overall I want to say it was pretty safe there, just use common sense and be aware of those around you and have a safe and enjoyable time!
Watch out for all the guys who are asking you for money or cigarettes. We were drinking something in front of the McDonalds in the centre of Porto! And each two minutes there was another one asking for money or cigarettes. Also the same people came back several times!