MOSCOW — Aleksei A. Navalny, the Russian opposition leader, said on Tuesday that a doctor told him he had lost 80 percent of the sight in one eye after suffering a chemical burn when an assailant threw a green liquid in his face last week.

The eye’s vision may improve, but the outlook is unclear, Mr. Navalny wrote on his website, citing his doctor.

Initially, the attack had appeared less serious; dousing opposition figures with green dye is a common occurrence and often attributed to pro-Kremlin activists. Typically, a type of topical medical disinfectant has been used. It is difficult to wash out but harmless.

Mr. Navalny, who has declared his intention to run in Russia’s presidential election next year, had already been splashed in the face with green dye once this spring, without any adverse affects.