Not on the street
Walking around a city centre with an open beer is not something a visitor should assume is legal.
A beer in a restaurant garden is normal. The same beer on a public square can be illegal. Here is the simple rule visitors need.
SHORT ANSWER
That includes places such as streets, squares, parks and public transport. The important exception is a place specifically intended for drinking, such as a restaurant, bar or licensed outdoor beer garden.
Walking around a city centre with an open beer is not something a visitor should assume is legal.
A bar or restaurant terrace intended for alcohol consumption is a normal legal setting.
PRACTICAL EXAMPLES
Do not assume a picnic beer is legal just because other people are sitting on the grass.
A public beach is not automatically a legal drinking area. Local/designated exceptions can exist, so check signs and local rules.
Drinking alcohol on public transport is prohibited.
If it is part of the licensed restaurant or bar area, drinking there is fine.
Polish Police guidance for foreigners states a limit of 0.02% blood alcohol. If you are driving, the simplest rule is not to drink.
FAQ
Generally no. Public consumption of alcohol is prohibited unless the place is specifically designated or covered by an allowed exception.
Generally no. Streets and public squares are examples of public places where alcohol consumption is prohibited.
Yes if you are sitting in the bar or restaurant's designated outdoor serving area. Simply standing outside on a public pavement is different.