Porto is a lovely place and I had no problems in any areas. An amazing place where I felt safe and could explore all day. lovely people, infrastructure, food, all of it.
My only word of caution is the poverty area near the cathedral. I wondered into this area by mistake (on my own) and ignored the graffiti above the arch that said "welcome to hell". I put it down to a single disgruntled soul but I turned the corner there was a mob of disgruntled souls, and I mean a mob. Quite scary and I've travelled alot.
They left me alone as I confidently walked past them (not being funny but I'm a big guy and not intimidated easily) but my concern would be to those who aren't my size. Young families, girls alone and in pairs.
These people had nothing to loose by asking me for an entry/exit fee. I mean what's the worst that can happen to them-they are already in hell by their own admission.
Porto is a safe city, but it has a dirty underbelly. Look under enough stones and you will see the homeless, the drug addicts and gangs of disaffected youth hanging around near the tourist sites, but out of sight of the heavy tourist police presence. This was particularly prevalent in the streets under and around the cathedral, but you can also find seedy and sleazy streets behind the Sao Bento station where the prostitutes hang out. I wasn't bothered by them, and I walked down the steep, narrow and maze like steps from the cathedral to the river without a single problem. Here Porto's poor, yet to be ousted through gentrification of the inner city, hide in the shadows, dogs running around uncontrollably, kids playing semi-naked in showers of hose-pipe water, windowless homes exposing decayed rooms and unwashed sheets.
As any important city , Porto has his own wild citizens... so just be aware where you going to... and do not do the tourist with the camera, the map and that ind of stuff...
I had some problems walking indeep into Vila Nova de Gaia...