Alexei Navalny was arrested Monday immediately following the Russian opposition leader's release from prison after serving a 30-day sentence for an unauthorized protest.
"Alexei Navalny was detained outside the detention centre," his spokeswoman, Kira Yarmysh, wrote on Twitter, adding that the politician was taken to a central Moscow police station.
Yarmysh said President Vladimir Putin's top critic was accused of violating a different protest law and faces up to 20 days in prison. He is due to appear in court later Monday, she added.
Navalny was in jail for a month for a protest he organised in January, violating laws which forbid any public event without authorisation.
The 42-year-old has accused the authorities of sending him to prison to keep him from holding another protest on September 9 against the government's retirement age hike plan that coincided with local elections.
Thousands of Russians across the country turned out despite his arrest and more than 1,000 people were arrested, according to the independent monitor OVD-info.
Navalny has criticised the pension reform that has led to a rare outburst of public anger in Putin's Russia.
Writing on Twitter, his associate Lyubov Sobol said he Navalny was detained again "because the authorities are now weaker than ever."
"The overwhelming majority of people are against raising the retirement age and (ruling) United Russia (party) was defeated in gubernatorial elections in key regions," she wrote.
"They are scared, panicking and taking revenge."
The Kremlin-backed plan to raise the state pension has seen Putin's approval ratings take a major hit.
About 3,000 people demonstrated in Moscow on Saturday against the reform in a protest organised by the Communist Party and approved by local authorities.
Public anger over the pension age hike and falling living standards as a result of tough Western sanctions has also led to rare election reverses for the Kremlin.
This month the Kremlin suffered a string of setbacks as United Russia candidates failed to secure victories in four regions, including in Primorsky Krai and Khabarovsk, and south Siberia's Khakasia in the far east.
On Sunday, a second round of gubernatorial elections was held in two regions -- Vladimir, 190 kilometres (120 miles) northeast of Moscow, and Khabarovsk but United Russia candidates were defeated by nationalists from the LDPR party.
In an unprecedented move, the second round run-off in Primorsky Krai was cancelled last week following protests over vote-rigging in favour of the Moscow-backed candidate and a re-run is expected to take place in three months.
Khakasia is yet to hold a run-off vote.
Navalny shot to prominence as an organiser of huge anti-Putin rallies that shook Russia in 2011 and 2012 following claims of vote-rigging in parliamentary polls.
His anti-corruption rhetoric is especially popular with younger people who follow his online channels and blogs.
He was barred from contesting Russia's presidential election in March and served a month in prison in June after organising demonstrations ahead of Putin's swearing-in ceremony for a fourth Kremlin term in May.
The Yale-educated lawyer has faced a string of charges since he became the leading opposition figure in Russia.
His spokeswoman said Navalny had been arrested nine time and had spent a total of 172 days in jail.