Since Sunday, the day before the inauguration, when the authorities broke up a permitted protest for still unclear reasons, the police have arrested at least 600 activists in Moscow, including protest leaders on multiple occasions for several hours. On Wednesday, a court sentenced Mr. Navalny and another protest leader, Sergei Udaltsov, to 15 days in jail for disobeying a police order.

“This is a peaceful protest against Putin’s inauguration, but it shouldn’t look like a protest,” Oleg, a programmer out at a park called Kitai Gorod, about a mile from the Kremlin, said late Monday evening, a mat furled under one arm. “That’s why we say we’re just here to relax, and that’s why I brought this mat.”

On Monday, the authorities answered the new tactics by sending a street-sweeping machine with a water nozzle to sprinkle Oleg, who gave only his first name for fear of arrest, and others who sat down. It worked, at least temporarily, and there were no arrests.

Two days later, amid throngs of tourists eating ice cream and posing for pictures outside the Kremlin gates, a group of mostly young people wearing white ribbons and carrying flowers suddenly broke into song. First it was a perestroika-era protest tune and then a World War II-era army song in honor of the anniversary of the Soviet Union’s victory over Nazi Germany.

Riot police officers in black helmets, jackboots and flak jackets moved in almost immediately, joined by a uniformed police officer with a megaphone, who began repeating in a steady monotone: “Dear citizens, your action is illegal. Disperse immediately.”

The group of singing youths simply walked away.

Also on Wednesday, a few hundred young people wearing white ribbons formed a flash mob in an unlikely spot: the middle of a Communist Party march to celebrate Victory Day. Because the Communists had a parade permit, they managed to walk nearly to the Kremlin walls while chanting “Down with Putin!”