Read in the late 1980s, this was a sci-fi novel about an alien invasion which quickly overpowered all opposition globally. The protagonist was a trusted go-between between humans and the invaders who eventually turned against his alien masters. He stole alien anti-gravity technology and used it to spray paint massive graffiti on public monuments e.g., Big Ben in London from memory. I seem to recall the graffiti was of a monk in a habit which may have been what the main character wore when completing his protests. Whilst the aliens were massively technically superior, I think in the end they left as they were unable to comprehend how anyone could or would betray them
This sounds like "Way of the Pilgrim" by Gordon R. Dickson.
When the giant, armored, alien Aalaag descend upon Earth with their impregnable defenses and irresistible weapons, the planet submits completely. Three years later, expert linguist Shane Evert, as one of the few humans able to produce an intelligible version of the alien speech, is translator, courier, and slave of the Aalaag head honcho, Lyt Ahn. On his official travels, Shane witnesses the execution of a fellow human. The incident moves him to a small show of defiance: he scratches on a nearby wall a line drawing of a pilgrim. Mere months later, to Shane's astonishment, the symbol has spread worldwide as a focus of human resistance. Then Shane tricks the Aalaag into releasing attractive resistance-worker Maria; her associates persuade Shane to help them, since only Shane knows how the Aalaag think (they regard humans as their cattle). He concocts a dubious plan to force the Aalaag to leave.
Eventually, the plan goes into effect: organized, unarmed mobs of people dressed in pilgrim robes, defying death, hurl themselves bodily against the Aalaag strongholds. So the Aalaag, convinced that their human cattle can never be civilized, leave in disgust.