More than 50 years after they played together in the streets, Ben Zion Blustein told an Old Bailey jury that he once overheard Mr Sawoniuk saying he enjoyed working as a policeman for the Nazis. "[Before the Nazi invasion of Belarus] he lived and earned his living from the Jews so his relationship must have been all right," Mr Blustein said. "[Afterwards] he was to behave cruelly whenever he wanted and with whomsoever he wanted."
Mr Blustein said he once watched as Mr Sawoniuk beat a young girl after she tried to smuggle potatoes into the Jewish ghetto created by the Nazis in the centre of Domachevo. Later he heard him saying to a fellow policeman: "At the moment, things are good for me. I want to live the way I wish to."
Speaking in Hebrew and addressing the court through a translator, he added: "I had known him since I was nine or ten. The school at which I studied was close to his house and he bred pigeons.
"As children we used to play and run. We bought pigeons from him and in the summer months we used to go and wash in the stream near his house ... I used to meet him almost daily."
Mr Blustein, 76, who now lives in Israel, said that within days of the Nazis invading Belarus, Mr Sawoniuk volunteered as a police officer. "He became a man of power, a master, a lord, and I was a Jew, who had taken away all rights that a person needs in order to survive," he said.
Mr Blustein was giving evidence to the jury after it returned from a visit last week to Domachevo, the town in Belarus in which Mr Sawoniuk is alleged to have murdered up to 20 Jews while working as a policeman during the Nazi occupation of 1941-44. Mr Sawoniuk, 77, from south London, denies all of the charges.
Earlier, Mr Blustein had outlined the terror of living as a Jew in Domachevo once the Germans overran the town in the summer of 1941. Forced to live in a ghetto and wear yellow patches on their chests and backs, Jews were subjected to curfews, rationing and regular beatings. Many were killed.
It was so bad that "if someone died, people were jealous", he said.
The first Jew to be murdered was shot by the Nazis just three days after the invasion, Mr Blustein said. The victim's name was Mendel Rubenstein.
"There was an SS unit and they wanted to create terror and panic among the Jews," Mr Blustein said when asked why the Nazis resorted to murder. "After they killed Rubenstein they got their hands on a Jew called Hershke Grynstein. He was a barber. They also got hold of his three sons. They shot the three sons and they forced the father to bury them.
"When the father started throwing earth on to one of his sons, one son was still alive and he said to his father, `What are you doing?I am still alive.' But with beating, they forced the father to bury him."
Mr Blustein also told how the Nazis murdered a leading rabbi and 40 of his followers. They had forced them to load a cart with sacks of flour before ordering them to pull it towards the river. "They shot them all," Mr Blustein added.
As the killings continued, many Jews tried to build hiding places in the ghetto. Mr Blustein said he built a shelter measuring two metres by one metre under the floor of his aunt's house, in which four of his family were to later hide.
Mr Sawoniuk, a former British Rail ticket collector, is alleged to have assisted the Nazis enthusiastically, organising "search and kill" operations for those who escaped the Nazi slaughter of 2,900 Jews. He is alleged to have fled Domachevo in 1944 when the Red Army counter-attacked and forced the Nazis back to Germany.
The trial continues.