MOSCOW -- Supporters of opposition leader Alexei Navalny rallied across Russia on Saturday, heeding his call to push authorities to let him enter the presidential race.

The wave of demonstrations on the 65th birthday of President Vladimir Putin comes as Navalny is serving a 20-day jail term for calling for an unsanctioned protest.

Navalny's headquarters said protests were to be held in nearly 80 Russian cities.

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Rallies numbering from a few dozen to a few hundred people were held in cities across Russia. Most of the demonstrations haven't been sanctioned by authorities, and police detained a few protesters and activists in some cities.

In Moscow, several hundred protesters, most of them young, gathered on Moscow's downtown Pushkinskaya Square, waving Russian flags and chanting "Russia will be free!" and "Free Navalny!"

Supporters of Russian opposition leader Alexei Navalny attend a rally in Moscow, Russia Oct. 7, 2017. SERGEI KARPUKHIN / REUTERS

Police warned the demonstrators that the rally hasn't been sanctioned and urged them to disperse, but didn't immediately move to break up the rally. Several people were detained.

Escorted by police, some protesters later walked down Tverskaya street toward the Kremlin, shouting "Putin, go away!" and "Future without Putin!" Police lines blocked them from approaching Red Square.

Police and the National Guard showed up in full force in downtown Moscow and in St. Petersburg, where a rally is planned for later Saturday.

Navalny has declared his intention to run for president in the March 2018 election, even though a criminal conviction that he calls politically motivated bars him from running. The 41-year-old anti-corruption crusader has organized several waves of protests this year, casting a challenge to the Kremlin.

Putin hasn't yet announced his intention to seek re-election, but he's widely expected to run. With his current approval ratings topping 80 percent, he is set to win another six-year term in a race against torpid veterans of past election campaigns, like Communist Party chief Gennady Zyuganov.

Navalny argued that a high level of support for Putin comes from the lack of real political competition and urged supporters to help him get registered for the race.

"The 86-percent approval rating exists in a political vacuum," he said. "It's like asking a person who has been fed with rutabaga through his entire life how eatable they find it and the rating will be quite high. Listen, there are other things, which are better than rutabaga."

The sarcastic analogy demonstrated Navalny's stinging style, which has recently helped him get broad support among the young.

Navalny has worked to expand his reach with videos exposing official corruption and YouTube live broadcasts. His documentary about Prime Minister Dmitry Medvedev's alleged ill-gotten wealth has been viewed nearly 25 million times since its release in March, helping galvanize protests.

Heeding his call, tens of thousands took to the streets in dozens of cities and towns across Russia in March and June, the biggest show of defiance since the 2011-2012 anti-government protests in Russia.