Russian police have made nearly 500 arrests at opposition rallies in the country's two main cities, including several well-known protest figures.
Opposition leader Alexei Navalny was among those picked up in Moscow on Monday evening, as he attended an unapproved rally near the Kremlin.
He and others have appeared in court, charged with offences that entail a fine or detention of up to 15 days.
The rallies were called to protest at sentences passed on other activists.
Seven people had received prison terms of up to four years on Monday, for rioting and attacking police at a demonstration against Vladimir Putin's inauguration for a third presidential term in May 2012, in Bolotnaya Square, Moscow.
Human rights organisation Amnesty International condemned the sentences as a "hideous injustice", at the end of a "show trial".
An eighth defendant, the only woman on trial, received a suspended sentence.
While the rallies on Monday in Moscow and St Petersburg were called to protest at the Bolotnaya sentences, some demonstrators also made shows of solidarity with the protesters in Ukraine, who brought down President Viktor Yanukovych last week.'Maidan'
Police arrested 420 people in Manege Square, under the walls of the Kremlin, hours after picking up some 200 people outside the court where the Bolotnaya defendants were being tried.
It appears that many of those detained near the court were released shortly afterwards, and at least some of them went to Manege Square for the bigger rally.
Among those detained in Moscow were Pussy Riot punk band members Nadezhda Tolokonnikova and Maria Alyokhina, both recently freed from prison under an amnesty, and former Deputy Prime Minister Boris Nemtsov.
Those held overnight were apparently being charged with either disobeying a police request or breaching the rules on holding rallies.
In St Petersburg, 60 people were arrested at a similar protest rally.
Outside the court in Moscow, some demonstrators had shouted "Maidan" - a reference to the square in Kiev where Ukrainian protesters camped out before finally toppling the country's elected president.
However, Mr Navalny distanced himself from events in Ukraine, saying that Russia had its own battle for freedom.
Russian state TV coverage of the Bolotnaya trial drew analogies with the unrest in Ukraine, the Associated Press notes.
"With the events on the Maidan as a background, it's even more obvious what all of this could have led to, had it not been for the government's clear, tough response," one unnamed TV presenter was quoted as saying.