A foreigner detained by the police pursuant according to Section 27 of Act No. 273/2008 Sb. or 326/1999 Sb. undergo a PCR test.
According to point I/17 (only in Czech) the Ministry of Health of the Czech Republic orders:
foreigners detained pursuant to Section 27 of Act No. 273/2008 Coll., on the Police of the Czech Republic, or pursuant to Act No. 326/1999 Coll., on the stay of foreigners on the territory of the Czech Republic and on the amendment of certain acts, as amended, to tolerate the collection of a biological sample for the purpose of detecting the presence of the COVID-19 disease and to provide medical personnel with the necessary assistance in carrying out this collection at the collection point of the health service provider or in the facility for the detention of foreigners.
Full text of the measure (only in Czech).