Leaving luggage in Porto

I'm planning to walk from Porto to Santiago in mid-June. I will be flying to Lisbon and taking bus or train to Porto. After my pilgrimage, I plan to return to Porto and spend a few more weeks in Portugal, so I am thinking of taking some extra clothing etc for the remaining time. Does anyone know if there is a place where I could stash a bag in Porto for a couple of weeks while I do my walk? Any information gratefully accepted.