Renewed protests are due to be held in Moscow and other Russian cities following Vladimir Putin's victory in last weekend's presidential election.
Authorities have given permission for up to 50,000 protesters to gather on one of central Moscow's large avenues.
A wave of protests was sparked last December by evidence that parliamentary elections had been rigged.
Similar allegations have surrounded the presidential vote, which saw Mr Putin win a third term.
Foreign states have accepted Mr Putin's victory but observers said the poll had been skewed in his favour.
The BBC's Richard Galpin in Moscow says some opposition leaders are playing down expectations of a huge turn-out on Saturday, partly because their movement has failed to stop Mr Putin securing another term.
Saturday's demonstration will be an important test of the appetite for continuing the street protests, he adds.
There are suggestions the opposition should change tactics and focus on setting up new political parties to try to challenge those in power.
The Moscow protest will take place on Novy Arbat, a vast avenue lined by 1960s skyscrapers.
"If we achieve a growth in the protests, if 100,000 people come out, it means we can quickly get results," said Sergei Udaltsov, one of the protest organisers.
"If there won't be that number, it means that there will be longer-lasting work."
He said the main slogan of Saturday's protest would be: "These were not elections. This is not a president."
Last Monday, a day after the election, police arrested about 250 protesters in Moscow and another 300 in St Petersburg.
Moscow officials say about 2,500 troops, police and civilian auxiliaries will be on duty in the city on Saturday, Itar-Tass news agency reported.
Mr Putin was re-elected for six years, having served two previous terms as president between 2000 and 2008.
On Friday, US President Barack Obama called Mr Putin from Air Force One "to congratulate him on his recent victory", a White House statement said.
Mr Obama said he looked forward to hosting Mr Putin at the G8 Summit in May at Camp David.