MOSCOW—A surge in Moscow’s protest movement this weekend has presented the biggest challenge in nearly a decade to President Vladimir Putin’s heavy-handed political system, giving Russians the chance to air an array of grievances over economic stagnation, lack of political choice and the Kremlin’s geopolitical isolation.

The military adventures of Mr. Putin, who celebrated 20 years in power last week, have turned Russia into a power to be reckoned with for foreign policy makers. But they haven’t translated into gains for average Russians, who have seen their living standards fall for five years in a row amid chronic economic problems aggravated by Western sanctions.

“The protests are about the deep disappointment Russians have in their government,” said Denis Volkov, a sociologist at Moscow-based pollster Levada. “It’s like the genie is out of the bottle and people can finally express their frustrations.”

The 50,000 protesters who converged on a wide boulevard only a mile from the Kremlin on Saturday proved that a movement that started with protests over municipal elections has turned into a platform for a wide swath of political interests—both local and national.

Russians have been hit by sclerotic economic growth, caused by corruption and an overdependence on oil and gas revenues. Average Russians have felt the pinch, causing consumer lending to balloon by as much as 46% last year. Some nationwide grocery chains have even introduced consumer loans for shoppers in grocery stores to allow Russians to put food on the table.