Mercado do Bolhao is the old market in the center of Porto.
There you will find many little stalls selling various things especially in terms of food and clothes.
The place has become quite run down in recent years though and the place seriously needs some investment if it is not to lose most clients.
The place is still worth visiting though for some local flavour, just be ready to see a market that is not flashy.
While you're in the city centre, take time to visit the Mercado do Bolhão, which sells a colourful array of flowers and local produce. Everything is beautifully presented and you get the impression that nothing ever changes here.
The market has an upper floor, from where you can get a great view of the activity below.
Open Mon to Fri 0800 - 1700; Sat 0800 - 1300