These are the female flowers of ilex crenata or more commonly known as the Japanese holly or box-leaved holly. I saw its beautiful black berries last winter near the Imperial Palace, but I didn't know at that time that it produces such beautiful tiny flowers.
Japanese hollies are dioecious, i.e., having separate male and female plants, so they have male and female flowers. I was able to find the female flowers, but couldn't find male ones...
