MOSCOW – Police detained anticorruption activist Aleksei Navalny as hundreds were arrested and thousands rallied in cities across Russia in the largest public demonstrations the country has seen in years.

Navalny, a gadfly crusader whose fight against graft has resonated with many Russians, was detained as he emerged with supporters from a subway station March 26 in central Moscow.

On Twitter, Roman Rubanov, the director of Navalny's nongovernmental foundation, posted videos of a crowd of supporters trying to prevent the van carrying Navalny from moving amid a heavy riot police presence.

Thousands of protesters crowded into the square, some of them chanting "shame!" and "defenders of thieves!" One protester mounted the base of a famous statue of poet Aleksandr Pushkin and raised a placard with "Putin 666." He was swiftly detained.

Several other protesters were detained as the riot police cleared the way for the van to pass through. Dozens of others were arrested at another square, not far away, amid a similarly heavy police presence and a helicopter circling over the crowd.

Navalny, who challenged Moscow’s mayor in 2013 elections and has announced his intention to run for the presidency in 2018, called on supporters to continue their protest without him.

More than 800 people were believed to be detained in Moscow alone, according to the the nongovernmental organization OVD-Info. City police did not immediately release any figures, but the state news agency TASS, citing an unnamed Moscow police source, said more than 500 people were arrested.

​Yury Kostyuk, 28, who works for a telecommunications company in a regional city, said he attended the rally because “the horrific amount of corruption and theft.”

“As they say, the fish rots from the head down. We're here to remove the head," Kostyuk told RFE/RL.

Yevgeny, an unemployed 51-year-old who declined to give his surname, said he turned out because he felt the laws applied only to the powerful.

"The problem is there are no laws and there is no justice. Even when there is the law, it doesn't apply to everyone. The slogan of the rally should be: the law applies to all," he told RFE/RL.

Combined with similar rallies in several cities across Russia, the demonstrations were among the largest since 2011 and 2012, when thousands rallied to criticize elections that were won by Russia’s ruling party and President Vladimir Putin.

The size of the March 26 protests was remarkable as well because they were unauthorized. Recent laws have tightened criminal punishment for protests not sanctioned by the city authorities.

Leonid Volkov, who heads Navalny’s foundation, said police officers were searching the foundation’s Moscow offices March 26, and had detained a number of foundation employees.

The rallies followed a report released by Navalny's foundation on March 2 accusing Prime Minister Dmitry Medvedev of using charities and NGOs to collect donations from tycoons and state banks and using the funds to buy expensive assets.

Navalny said on his website that protests would be held in 99 Russian cities, but local authorities refused to give official permission in 72 of them. Still, the demonstrations attracted crowds of hundreds or thousands in most large cities across the country, according to local media.

In the port city of Vladivostok, home to the Russian Pacific Fleet, police detained at least 30 people. Local Russian media outlets reported around 1,000 people came out to the rally, which was not authorized by the city administration.

Some 2,000 people gathered in the Siberian city of Novosibirsk, where the protest was authorized by the authorities. Protesters held signs that said “No to corruption,” and images of yellow rubber ducks, after reports that Medvedev had a special house for a duck on one of his properties.

​According to local media around 1,500 people came out in the Siberian cities of Krasnoyarsk and Omsk. In Yuzhno-Sakhalinsk, at least five were detained at the rally and nine others afterward. Six people were detained in Komsomolsk-on-Amur.

In what may be an indication of the depth of Russians' unhappiness with rampant corruption, rallies were also held in smaller provincial cities, such as Komsomolsk-on-Amur, home to fighter jet manufacturer Sukhoi. Six people were detained there.

And in Makhachkala, the capital of the North Caucasus region of Daghestan -- a region not known for antigovernment protests -- more than a dozen people were detained after police told organizers the rally was unsanctioned.

Navalny announced in December that he would run for president next March when Putin is widely expected to seek a new six-year term.

Russian authorities have said Navalny will be barred from the ballot if a conviction on charges of financial crimes is upheld on appeal. But he has pushed ahead with his campaign.

Navalny has said the two previous convictions in two separate cases were politically motivated punishment for his opposition to Putin.

With reporting by RFE/RL’s Russian Service, Current Time TV, Interfax, and AP