The Putin critic had served a 20-day jail term for organising protests against the country's leader
Russian opposition leader and anti-Putin campaigner Alexei Navalny declared "I'm out" after he was released from a detention centre in Moscow.
The 41-year-old lawyer, who plans to run in next year's presidential election, was freed after serving a 20-day jail term for organising protests against the country's leader.
Mr Navalny has posted a picture on Instagram of himself on a street, with the words: "Hi. I'm out."
He joked that while in detention he had read 20 books, learnt a few words of the Kyrgyz language and drunk 80 litres of tea.
He had been detained late last month while on his way to a political rally.
Mr Navalny, the head of the politically centrist Progress Party, which campaigns against corruption, is one of the country's most prominent critics of Mr Putin.
His release was arranged by police to evade many journalists waiting outside the centre.
Mr Navalny said he was "ready to work" and was heading to a rally later in the southern city of Astrakhan, which authorities have allowed to go ahead.
Привет. Я вышел. За 20 суток прочитал 20 книг, выучил несколько слов на киргизском, выпил литров 80 чая и не менее 60 раз прослушал песню "Дай мне слово ведь я тамада" по радио. Готов к работе. Сегодня вечером выступаю на митинге в Астрахани: парк "Дружба" (бывший парк АТРЗ), остановка "Жилгородок". В 18-00. Приходите. #спецприемник #арест #астрахань #тамада #рахмат #нучечайку
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His supporters hung a banner from a bridge close to the Kremlin that read: "It's time to get rid of Putin and time to elect Navalny."
During his jail term, his backers held rallies on 7 October - Mr Putin's birthday - with more than 270 detained nationwide.
The rallies have attracted thousands of people, and have recently focused on broadening support for opposition politics in more remote regions of Russia.
It is not the first time this year that Mr Navalny has been detained for organising unauthorised anti-Putin protests.
He has previously served sentences of 15, 25 and 20 days.
In June, he was barred from standing in the presidential election after being found guilty of embezzlement.
The contest has not officially begun and Mr Putin has not yet declared whether he will be taking part in the March 2018 election.
But he is widely expected to join the race and win another six-year term - keeping him in power until 2024.
The contest hotted up this week when TV star and socialite Ksenia Sobchak announced on Wednesday she would be standing.