Carrying flowers and candles, the demonstrators offered a one-minute prayer in front of the Israeli embassy, with a Buddhist monk ringing a bell for the souls of the victims.
They placed a letter of protest in the embassy's mailbox hours after delivering a separate letter to Japan's foreign ministry urging it to put pressure on Israel. Japan is a major donor to the Middle East peace process.
Israel attacked the Gaza Strip Saturday in retaliation for rocket fire from the territory, killing at least 360 Palestinians and wounding 1,690.
The banner say "Don't kill children"