MOSCOW — Russia’s opposition, often written off by critics as a small and irrelevant coterie of privileged urbanites, put on an impressive nationwide show of strength Sunday with scores of protest rallies spanning the vast country.

Hundreds of protesters were arrested, including Alexei Navalny, the anti-corruption campaigner who is President Vladimir Putin’s most prominent critic.

It was the biggest show of defiance since the 2011-2012 wave of demonstrations that rattled the Kremlin and led to harsh new laws aimed at suppressing dissent. Almost all of Sunday’s rallies were unsanctioned, but thousands braved the prospect of arrests to gather in cities from the Far East port of Vladivostok to the “window on the West” of St. Petersburg.

An organization that monitors Russian political repression, OVD-Info, said it counted more than 800 people arrested in the Moscow demonstrations alone. That number could not be confirmed and state news agency Tass cited Moscow police as saying there were about 500 arrests.

Navalny, who was arrested while walking from a nearby subway station to the demonstration at Moscow’s iconic Pushkin Square, was the driving force of the demonstrations. He called for them after his Foundation for Fighting Corruption released a report contending that Prime Minister Dmitry Medvedev has amassed a collection of mansions, yachts and vineyards.

Navalny is a persistent thorn in the Kremlin’s side. He has served several short jail terms after arrests in previous protests and has twice been convicted in a fraud case, but given a suspended sentence. He intends to run for president in 2018 – an election in which Putin is widely expected to run for another term – even though the conviction technically disqualifies him. Putin has dominated Russian political life, as president or prime minister, since 2000.

No overall figures on arrests or protest attendance were available. Some Russian state news media gave relatively cursory reports on the demonstrations; the state news TV channel Rossiya-24 ignored them altogether in evening broadcasts.

Police estimated the Moscow crowd at about 7,000, but it could have been larger. The 2.5-acre Pushkin Square was densely crowded, as were sidewalks on the adjacent Tverskaya Street.

In St. Petersburg, about 5,000 protesters assembled in the Mars Field park, shouting slogans including “Putin resign!” and “Down with the thieves in the Kremlin!”

Russia’s beleaguered opposition is often seen as primarily a phenomenon of a Westernized urban elite, but Sunday’s protests included gatherings in places far from cosmopolitan centers, such as Siberia’s Chita and Barnaul.

There were no comments reported from Putin, Medvedev or other top Russian politicians.