"There are no signs of other injury on the body," the ministry said in a statement, indicating a probable suicide.
It said the man was from the Volyn region in western Ukraine, and his corpse has been cut down and taken away.
The Kiev municipality unsuccessfully tried to put up the tree on Independence Square -- the Maidan -- at the start of the demonstrations in what was seen as an attempt to put off the protesters.
Hundreds of protesters camp out in tents on the square, while thousands more sleep in occupied official buildings nearby.
Many have come from western Ukraine where the opposition to President Viktor Yanukovych is particularly trenchant.