Navalny’s health is reportedly in severe decline after three weeks of hunger striking, according to a group that monitors political detentions.
He was arrested back in January after accusing President Putin and his allies of stealing billions.
About a month later, he was jailed for violating parole from a 2014 sentence for embezzlement. He said the case was politically motivated.
The largest of Wednesday’s protests took place in Moscow, where thousands marched through the city.
Some of the people arrested were seized before the protests even began, including to top Navalny associates in Moscow.
Navalny’s team called for the unsanctioned demonstrations after weekend reports that his health is deteriorating and his life was in danger.
“The situation with Alexei is indeed critical, and so we moved up the day of the mass protests,” Vladimir Ashurkov, a close Navalny ally and executive director of the Foundation for Fighting Corruption, said.
“Alexei’s health has sharply deteriorated, and he is in a rather critical condition. Doctors are saying that judging by his test (results), he should be admitted into intensive care.”
Navalny’s organization called for the Moscow protesters to assemble on Manezh Square, just outside the Kremlin walls, but police blocked it off.
Instead, a large crowd gathered at the nearby Russian State Library and another lined Tverskaya Street, a main avenue that leads to the square. Both groups then moved through the streets.
“How can you not come out if a person is being murdered — and not just him. There are so many political prisoners,” said Nina Skvortsova, a Moscow protester.
In St. Petersburg, police blocked off Palace Square, the vast space outside the Hermitage museum and protesters instead crowded along nearby Nevsky Prospekt.
It was unclear if the demonstrations matched the size and intensity of nationwide protests that broke out in January after Navalny, President Vladimir Putin’s most prominent opponent, was arrested accusing the Russian president and his allies of stealing billions.
Turnout estimates varied widely: Moscow police said 6,000 people demonstrated in the capital, while an observer told Navalny’s YouTube channel that the crowd was about 60,000.
The OVD-Info group, which monitors political arrests and provides legal advice, said at least 1,496 people were arrested in 82 cities — the largest tally being nearly 600 in St. Petersburg.
Navalny’s team called the nationwide protests for the same day that Putin gave his annual state-of-the-nation address.
In his speech, he denounced foreign governments’ alleged attempts to impose their will on Russia.
Putin, who never publicly uses Navalny’s name, did not specify to whom the denunciation referred, but Western governments have been harshly critical of Navalny’s treatment and have called for his release.