In 1945, he was transferred to the newly formed Ministry of Public Security of Poland (officially "MBP", but commonly abbreviated to "UB"). In his work, Światło, like many other communist secret police agents, used torture and forgery. He was involved in arresting hundreds of members of Polish underground organization, Armia Krajowa, its leadership (the Trial of the Sixteen) and falsifying of the 1946 Polish referendum. In time he was promoted to Lieutenant colonel (pol.Podpułkownik) and served in various offices and departments, eventually in 1951 ending up in the 10th Department, where he was one of the leading officers. The 10th Department was responsible for handling the Party members themselves. He received orders personally from the First Secretary of the Polish United Workers' Party Bolesław Bierut, and arrested such notable people as politicians Władysław Gomułka and Marian Spychalski, General Michał Rola-Żymierski and Cardinal Stefan Wyszyński. He had access - sometimes unique - to many secret documents. He interrogated Noel Field on 27 August 1949 in Budapest as well as his brother, Herman Field (a US citizen who went to Poland to look for his brother). Herman would be secretly imprisoned for five years, until the information on him was revealed - by Światło himself.