Thousands of Russians protested endemic corruption in the Russian government on Sunday, defying thinly-veiled threats issued by the Kremlin that it would "bear no responsibility for any possible negative consequences" for demonstrators.

The Russian news agency Tass reported that more than 500 people were arrested in Moscow alone, making Sunday the most massive detention of protesters during the presidency of Vladimir Putin. The protests in more than 80 cities come a year before a presidential election in which Putin is expected to run for a fourth term in office.

Among those arrested was Russian opposition leader and protest organizer Alexey Navalny, who has vowed to oppose Putin in the 2018 presidential elections, despite having been barred from doing so by the government.

На Площади Революции красиво! #ДимонОтветит pic.twitter.com/HB9glGpfDr

— Philipp Kireev (@mynameisphiIipp) March 26, 2017

Caption: Police waiting for protesters in Moscow

Alexey Navalny called for Sunday's demonstrations after the Kremlin failed to respond to his report accusing Prime Minister Dmitry Medvedev of using charities and NGOs to collect donations from tycoons and state banks. In the report, Navalny accused the prime minister of using the funds to buy vineyards, luxury yachts, and opulent mansions. The Prime Minister's press secretary Natalia Timakova and presidential spokesman Dmitry Peskov declined to comment on the accusations.

Russia ranked 131th place out of 176 countries in the 2016 Corruption Perception Index of Transparency International.

"People surrounded the police car with Navalny and did not let them pass," tweeted Ilya Varlamov, who witnessed the Navalny's arrest.

Люди окружили автозак с Навальным и не дают ему проехать#ДимонОтветит онлайн: https://t.co/Uqrpaeot1t pic.twitter.com/BAJ9yUQCiu

— Илья Варламов (@varlamov) March 26, 2017

Marches in more than 20 cities were sanctioned by municipal officials, while another 60 other protests were denied government approval. The nation-wide movement appeared to be one of the largest coordinated outpourings of dissatisfaction since the massive 2011-12 demonstrations that followed a fraud-tainted parliamentary election.

"God, what's happening on the streets. People go for freedom, knowing that they will be detained! I'm inspired," tweeted Diana Yusupova.

Боже,что происходит на улицах. Люди выходят за свободу ,понимая ,что будут задержаны! Я вдохновляюсь #димонответит

— Diana Yusupova (@Muza_Chehova) March 26, 2017

В Челябинске толпа дошла до главной площади и здания регионального Заксобрания. С удвоенной силой кричат "Долой коррупцию!" и "Путин вор!". pic.twitter.com/gzbF9dJJ5B

— ௵_Надежда (@nadezdanba) March 26, 2017

Translation: "In Chelyabinsk, the crowd reached the main square and the building of the regional Legislative Assembly. With redoubled force shout "Down with corruption!" And "Putin is a thief!"

Владивосток: одни против коррупции, другие - на её защите. pic.twitter.com/f7quPpU7d5

— Alexey Navalny (@navalny) March 26, 2017

Translation: "Vladivostok: some against corruption, others - on its protection"

Users on Vkontakte, the Russian version of Facebook, wrote that Russian authorities took special measures to prevent people from going out into the streets. In the city of Belgorod, high schools scheduled classes on Sunday, which is normally a week-end day without school. In Komsomolsk-on-Amur, the police cordoned off the planned location of the protest location, citing a bomb threat. 

In response, demonstrators gathered in other locations.

2 человека задержано за плакат pic.twitter.com/jd3dA6GHm8

— Dave Frenkel (@merr1k) March 26, 2017

Translation: "2 people detained for protesting" in St. Petersburg, before protests even began.

#Москва на Белорусской уже 15 человек задержали. Хватают за плакаты Тут уже несколько сот активистов. #Димонответит pic.twitter.com/bHFcs5TSsU

— Денис Стяжкин (@styazshkin) March 26, 2017

Translation: "At the Belaruskaya station (Moscow) 15 people were detained. There are already several hundred activists."