MOSCOW — The shot opens at the top of a flight of stairs and zooms in shakily on a gray-haired man, who sits at a desk furtively checking off what appear to be ballots — a stack of them.

The video is shot with the grain and chop of an amateur. But it is apparently sharp enough.

“A big hello to you,” says the cameraman, Yegor Duda, a 33-year-old volunteer election observer. “This is a violation of the criminal code. The chairman of the electoral commission is filling out ballots. Everything has been captured on the video camera,” he said.

Mr. Duda raced home and uploaded the clip to YouTube. Though just three minutes long, it quickly became an election-day sensation, helping fuel a major demonstration of as many as 5,000 people on Monday evening in central Moscow. They chanted “Russia without Putin!” and “Putin is a thief.” Several hundred were arrested, including two major opposition leaders.

Valentin Gorbunov, the head of the Moscow City Elections Commission, confirmed the substance of the video and announced that Russian investigators had opened a case into ballot tampering by the head at Polling Place No. 2501, where the episode occurred, Russian news agencies reported Monday.