He said on Wednesday that he now believed he was well enough to return to Russia. He said he planned to travel on the low-cost airline Pobeda and that he would arrive in Moscow on Sunday.
“Come meet me!” he said.
Within days of emerging from a medically induced coma at the Charité hospital in Berlin in September, Mr. Navalny pledged to return to Russia. But his surprise announcement on Wednesday about the timing of that return jolted Russian politics — setting up a high-stakes decision for the Kremlin on how to respond.
Last month, working with the open-source investigative organization Bellingcat, Mr. Navalny released two YouTube videos documenting an elaborate plot by Russia’s domestic intelligence service, the F.S.B., to kill him. The videos have been viewed a total of 45 million times.
At the same time, the Kremlin raised the pressure on Mr. Navalny, signaling that he would end up in jail if he returned to Russia. President Vladimir V. Putin described Mr. Navalny as a C.I.A. asset and quipped that if Russian agents had wanted to kill the opposition leader, “they would have probably finished the job.”
But imprisoning the opposition leader would carry risks for the Kremlin because the move could set off protests, and, by announcing his imminent return, Mr. Navalny appears to be calling Mr. Putin’s bluff. An ally of Mr. Navalny, Lyubov Sobol, was jailed in Moscow for 48 hours in December, then released.