In Czech Republic there is an exception for EU citizens: they must register
afterwithin 30 days and there's no penalty for failing to do so. – JonathanReez♦ ↵ Feb 18 '16 at 10:35
What's the point in registering with the police when visiting Czech Republic?
For an official source: https://www.policie.cz/clanek/reporting-of-the-place-of-aliens-residence.aspx
An EU citizen is obliged, within 30 days from his/her entry into the Territory, to report the place of his/her residence in the Territory to the police if his/her expected stay is to be longer than 30 days; this obligation also applies to an EU citizen's family member if that EU citizen is staying in the Territory. The obligation to report the place of residence to the police does not apply to an alien who has fulfilled this obligation with the accommodation provider.
Another official source: https://www.mvcr.cz/mvcren/article/entering-the-czech-republic.aspx:
One of the obligations ensuing from the Act on the Residence of Foreign Nationals (pdf, 1 MB) for citizens of the EU, Iceland, Norway, Liechtenstein, or Switzerland is the "reporting requirement" in the event that the length of the intended stay in the Czech Republic is longer than 30 days. In this situation, within 30 days of entering the Czech Republic, an individual is required to report his/her presence to the appropriate Foreign Police Department that holds jurisdiction in the location of his/her stay in the Czech Republic.