Navalny’s wife, Yulia, reported news of his detention at his home in Moscow on his official Twitter feed. She also posted a picture showing police officers at the scene.
Meanwhile Navalny’s YouTube channel, which had been broadcasting live to more than 50,000 people as protests across Russia got underway, lost light and sound in the studio.
Around 250 people were arrested at the opposition rallies in Russia’s capital and second city on Monday according to OVD — an independent group monitoring arrests.
OVD reported that 121 people had been arrested in Moscow and 137 in St. Petersburg.
Hours earlier, authorities had declared that the planned rally in the Russian capital was illegal.
“We warn that any attempts to hold an illegal event on the Tverskaya Street, Moscow is a direct violation of the law,” the Prosecutor General said in a statement.
“Law enforcement agencies will be forced to take all necessary measures to stop provocations, mass unrest or any actions leading to a violation of public security, creating conditions for threatening the life and health of citizens.”
Nearly 200 rallies were planned for towns and cities to mark the Russia Day public holiday.
Navalny — — who plans to run against Vladimir Putin in next year’s presidential election — said earlier they would go ahead regardless of whether the government allowed them or not. He planned to attend the Moscow protest before he was detained.
Photographs and videos posted on Twitter showed large numbers of police at Tverskaya Street in the capital, a main thoroughfare near the Kremlin. The protesters were shouting “Putin is a thief,” “Putin out” and “Russia without thieves.” Pictures on social media showed police putting gas masks on.
Navalny’s press secretary, Kira Yarmysh, posted on Twitter that tear gas had been used against her.
Meanwhile there was also a large police presence in St. Petersburg where a protest was taking place at the Field of Mars park, where protesters were chanting “shame”.
Navalny, 41, has been mobilizing support on social networks, and hopes the rallies will rattle the Kremlin, as those held earlier this year did.
In March, thousands joined protests in almost 100 cities across Russia, angered by a report Navalny published accusing Prime Minister Dmitry Medvedev of corruption. Navalny was arrested, along with hundreds of others, and was jailed for 15 days after being convicted of disobeying a police officer.
His election campaign comes despite him being convicted of embezzlement and given a suspended sentence in February. Russian laws prohibit convicted people from running for office. Navalny says the charges against him are politically motivated.