There is a common belief in Czech society that most of the prisoners in Czech Republic are Roma people. Wikipedia backs it up with a vague reference to a book from 1998:
According to Říčan (1998), Romani make up more than 60% of Czech prisoners and about 50% habitual offenders.
Is the above statement true as of 2017?
This is going to be a pretty disappointing answer, but here are various data points that might help. (Given the bickering in the comments to the question, I'll use the term "Romani" and hope for the best!)
Unfortunately, there is no official data that directly answers this question. According to 2015 data from the Prison Service of the Czech Republic, 540 or about 25% of pre-trial detainees were "foreigners". The number of foreigners "in prison" was 1057, about 6% of the total sentenced population. Annual reports give more detail on the foreign citizen populations of Czech prisons, but make it clear that foreign status is based on citizenship and makes no mention of Romani people.
According to the World Prison Brief, from 2000-2016, the total prison population in the Czech Republic has fluctuated, very roughly, between about 16,000 and 23,000 people. In order to constitute 60% of that, the hypothetical Romani prison population would have had to be around 9,600 to 13,800.
The March 2001 census (see Table 3 here) enumerated only 11,859 Romani or 0.12% of the total population. But such numbers are widely considered inaccurate. The European Commission gives estimates of 150,000 to 200,000 Romani people in the Czech Republic around 2012. Another estimate of the total population reported by The Economist in 2013 was 250,000. If we go with this most generous and recent estimate, then the 60% figure would imply that around 3.8% to 5.5% of the total Romani population in the Czech Republic would hypothetically be incarcerated.
According to a 2011 survey funded by the European Commission, nearly 40% of Romani adults in the Czech Republic considered themselves unemployed as compareed to about 10% of the non-Romani population. Other estimates of Romani unemployment in the Czech Republic vary wildly from about 20% up to 90%, but nobody argues that it is much higher than the general population.
Finally, we can look at relevant estimates of the share of Romani people among the prison populations of other countries. Here is a staggering estimate of 90% in 1990 from Bulgaria, another of 50% in 2008 in Hungary, and another of 5% ("Gypsy, Romany or Traveller") for 2012-2013 in the UK.
