Russian police have broken up an opposition rally, arresting activists for the second day running.
Police detained about 150 people in St Petersburg, including opposition leader Boris Nemtsov - who was later freed.
Another opposition figure, former chess champion Garry Kasparov, was arrested at a rally in Moscow on Saturday.
Both protests were organised by opposition group The Other Russia. The White House and the Council of Europe have expressed concern at the arrests.
Mr Kasparov was jailed for five days for leading an unauthorised march.
The Other Russia brings together a broad coalition of mainstream politicians, leftists and nationalists, all of whom are opposed to the Kremlin.
It accuses the government of crushing dissent ahead of elections.
White House spokesman Gordon Johndroe said on Sunday that the US was concerned at the "aggressive tactics" used by Russian authorities.
He called for arrested opposition figures "to be given proper access to legal counsel and fair treatment in processing".
The secretary general of the Council of Europe, an EU human rights watchdog, also expressed concern, the news agency AFP reported.
Several hundred people took part in the St Petersburg protest - much fewer than Saturday's gathering of about 3,000 people in Moscow.
Activists holding white flowers met near the headquarters of the liberal Yabloko party, and headed to the site of the unauthorised rally.
Mr Nemtsov, a leader of the opposition SPS party and a candidate in presidential elections due in March, addressed the crowd.
"They have forbidden us from discussing Putin," he said.
"But we have come here today to ask Mr Putin and the authorities why is there so much corruption in the country?"
Riot police reportedly moved in when some protesters unfurled banners of the banned National Bolshevik Party and marched towards the Winter Palace, the residence of the tsars.
Correspondents put the number of people taken away in police vans at about 200.
Mr Nemtsov was among them - but was subsequently released from custody.
The parliamentary poll to be held on 2 December is widely expected to be won by President Vladimir Putin's supporters.