The pictures are almost all reproductions of male nudes sketched by 19th-century Russian painters; the crude and ominous captions scrawled beneath them in red or blue pencil are unmistakeably in the hand of Joseph Stalin.

The set of 19 pictures, never before seen in public, went on display at a Moscow gallery yesterday, ready for the 130th anniversary of Stalin's s birth on Monday. Theyoffer an extraordinary insight into the mind of one of the 20th century's most controversial characters.

Stalin annotated the pictures with a variety of remarks, varying from laddish humour to thoughtful aphorisms. In one picture, he has scribbled out the genital area with red pencil; in another he has drawn a large blue cross across the man's torso, but in most of the portraits, he restricts himself to captions in the bottom corner, some of them signed. Next to one picture of a pensive nude, Stalin has written, "One thinking fool is worse than 10 enemies".

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Others have messages for Bolshevik comrades, both dead and alive: "Radek, you ginger bastard, if you hadn't pissed into the wind, if you hadn't been so bad, you'd still be alive", reads one caption, placed alongside a muscular male nude drawn from the back by Vasily Surikov, a famous 19th-century Russian painter.

Karl Radek was a leading activist about the time of the Russian Revolution, who had several disagreements with Stalin in the 1920s but was readmitted to the party in 1930 and helped draft Stalin's constitution of 1936. He fell out of favour the following year, and was sentenced to 10 years of hard labour. Historians believe he was executed in 1939 on Stalin's orders. The aggressive yet slightly lamenting tone suggests Stalin may have been tormented by the murders of some of his former comrades.

"Some people suggested this series might show Stalin had homosexual inclinations," said Viktor Turshchatov, a Russian journalist who helped organise the exhibition. "But to me it emphasises how alone this man was. You get a real sense of how solitary and isolated he felt."

Some of the captions show a crude, sexualised sense of humour. Another man is pictured naked from the back, standing against a wall with one hand appearing to reach down to his genitals. In the bottom corner, in red pencil, Stalin has scrawled, "You need to work, not wank. Time for re-education". Another shows a naked man standing over a prostrate naked woman. Stalin has captioned this one, "Idiot!!! You've completely forgotten what to do". Most of the captions are in Russian, but two portraits, one of a male nude and one female, both drawn from the rear, have captions in Stalin's native Georgian.

Nikita Petrov, a historian who has studied Soviet archives extensively, said he believed the captions were most likely written at some point between 1939 and 1946. It is unclear whether they were written in the presence of others or if they were private observations that Stalin wrote just to amuse himself.

In the early years of the Soviet Union, there was a tradition of top Bolsheviks drawing cartoons of each other during meetings, and passing each other notes with crude jokes written on them. But this practice dried up as the 1930s progressed: the leadership began to dwindle because of Stalin's ruthless purges, and humour was in short supply. "It displays a crude, soldier's humour and is very loutish," said Mr Petrov. "Your sense of humour says a lot about what kind of person you are. These captions show Stalin wasn't just malicious and primitive, but that he was also very dangerous."

Handwriting specialists have confirmedthewriting is Stalin's, but much remains unclear where the pictures have been since his death 1953. The exhibition organisers said the images had been held in a private collection by a family who assumed one of their ancestors had worked in Stalin's security, but tthe family did not want to be named.

"Usually, an exhibition comes when investigations into the works displayed have been completed," said Mr Turshchatov. "But in this case, we are hoping that the exhibition of these mysterious pictures will be just the start, and people who know the full story behind them might come forward."