In angry mass protests and subdued smaller gatherings, Chinese citizens have taken to the streets to protest the landing by Japanese activists on some barren islands that are claimed by both countries. Protesters in about a dozen cities on Sunday vented their anger at some Japanese adventurers — including conservative members of Parliament and other politicians — who swam ashore and planted national flags on a craggy chunk of the Senkaku island group. Known as the Diaoyu islands in China, they are also claimed by Taiwan.