The fans of Widzew and LKS are no danger to you, unless you say you like/ dislike one of the teams or have a scarf or t-shirt with teams symbols on you (which can be bought in a store for tourists).
Piotrkowska is NOT dangerous at all in the evening as well as at night. There are cameras and police patrols on every corner. It's actually the safest street in the center of the city. I strongly recommend going for a walk there.
You shold, however, beware surrounding streets: especially Pomorska, Traugutta, Moniuszki etc. There are no police patrols there and these are the places where the poorest people live.
Do not park your car in a parking on Narutowicza street after 6 o'clock during the week. Officially you dont have to pay for it after 6 o'clock, but after that time some low-lives gather and make you pay for "guarding" (or rather not smashing your windshield) your car. If you resist to pay, they will stand in your way, not letting you to leave the parking and damage your car.
Lodz has two major football teams, Widzew (pron. Vidzev) and LKS. The fans of these two teams tend to dislike each other and often fight when the two teams meet. Beware of Piotrkoska and surrounding areas when these two teams are playing against each other. It is also worth checking at the local tourist information office when these two teams play against each other.
Although Lodz has a good nightlife, walking down Piotrkowska at night can be dangerous, as gangs of thugs often threaten men walking on their own at night. If in doubt, flag down a taxi or rickshaw.
There are some police about, but it seems there are hardly enough.