In his letter, Mr. Snowden, 30, also appealed for clemency. He said his disclosures about American intelligence activity at home and abroad, which he called “systematic violations of law by my government that created a moral duty to act,” have had positive effects.

Yet “my government continues to treat dissent as defection, and seeks to criminalize political speech with felony charges that provide no defense,” Mr. Snowden wrote. “However, speaking the truth is not a crime. I am confident that with the support of the international community, the government of the United States will abandon this harmful behavior.”

Mr. Ströbele, 74, is a seasoned left-wing defense lawyer and the longest-serving member of the parliamentary committee that oversees German intelligence. At a packed news conference after his return to Berlin, he said he was contacted about going to Moscow late last week after the German government said Chancellor Angela Merkel’s cellphone might have been tapped by American intelligence agents. He declined to elaborate, but said he has had no dealings with the Russian authorities or the German Embassy in Moscow.