Born on 19 August 1896 in Nowy Sącz, Austrian Galicia, Berek studied medicine at the Jagiellonian University in Kraków.
In May 1918 he joined Polish Military Organisation, and in November of the same year he fought in the Battle of Lemberg, as leader of a company in 5th Legions' Infantry Regiment.
In December 1926 he was transferred back to 9th Legions Infantry Regiment, and in the period May 1929 – September 1931, he attended Wyższa Szkoła Wojenna in Warsaw.
After completing his officer course, he was chief of staff of the 4th Infantry Division, stationed in Toruń (1931–1934), and in 1934 – 36, he commanded 1st Rifle Battalion in Chojnice.
Promoted to Lieutenant Colonel on 1 January 1935, Berek served at the Polish General Staff (October 1936 – July 1939).