In a speech today, Evans said MI5 had identified 2,000 al Qaeda-connected individuals "who we believe pose a direct threat to national security and public safety."
Individuals as young as 15 and 16 have been implicated in terrorist-related activity, according to Evans, who said the extremists are "methodically and intentionally targeting young people and children in the U.K."
As for the Russians, Evans said "there has been no decrease" in Russian intelligence officers carrying out covert activities in Britain. MI5 is devoting significant resources, Evans said, to countering "unreconstructed attempts by Russian, and others, to spy on the U.K."
His comments are considered an important reflection of the current threat as seen by British officials.
Evans speaks only with "the concurrence" of the home secretary, according to officials.
His speech was delivered Monday to the Society of Editors Annual Conference in Manchester.