Russian socialite Ksenia Sobchak is to stand in the country’s presidential election in March, when Vladimir Putin is widely expected to run again.
Ms Sobchak conceded she was an unlikely candidate and said she supported opposition leader Alexei Navalny, who is barred from standing.
However he had warned her not to stand and some commentators now predict an opposition split.
The Kremlin welcomed her candidacy, saying it was fully constitutional.
Mr Navalny is currently serving a 20-day prison sentence for his role in organising “unsanctioned” protests.
He is banned from the election because of a conviction for fraud which he says was fabricated.
Sarah Rainsford, BBC News, Moscow
This announcement was a long time rumoured, so Alexei Navalny made his views clear last month shortly before his latest arrest.
The anti-corruption blogger dismissed Ksenia Sobchak as a Kremlin stooge, a “liberal laughing stock” and an opposition “caricature”, brought in to lend legitimacy to a sham vote. He scorned her as a showbiz celebrity, only seeking more social media “likes” and followers.
Ms Sobchak herself denies she is a spoiler, saying she will step down if Mr Navalny himself is allowed to run for president. That scenario looks highly improbable though.
So the socialite-turned-journalist has promised to be a “loud speaker” for those fed up with the lies, theft and corruption of their leaders. It is language that she borrows quite heavily from Mr Navalny.
She would be the first woman candidate in 14 years, a point which her campaign video in a kitchen underscores. But Ms Sobchak also has the “Marmite” effect: she is as unpalatable to as many people as she attracts.
And crucially, it’s not clear how far she’d actually go in criticising Vladimir Putin himself, a close family friend since her childhood.
Ms Sobchak, a journalist and TV presenter, called for the bar on him standing to be lifted.
She said she wanted to be “a mouthpiece for those who cannot be candidates”.
“I am against revolution but I am a good middleman and organiser,” Ms Sobchak wrote in a letter published on the website of Vedomosti business daily.
Russia’s election campaign starts around 7 December, when political parties are expected to hold congresses to nominate their candidates.
A Russian citizen not backed by a political party has the right to register as an independent presidential candidate provided he or she collects at least 300,000 signatures.
President Putin, who first took office as president in 2000, has not announced yet whether he will stand again.
Who is Ksenia Sobchak?
- Aged 35, she is the daughter of Anatoly Sobchak, the late mayor of Saint Petersburg who once employed little-known former KGB agent Vladimir Putin
- Nicknamed “Russia’s Paris Hilton”, she won fame by presenting a popular dating reality show called Dom-2
- Denigrated by some as a party girl, she also gained attention for once presenting another reality TV show from a bubble bath
- She raised eyebrows by joining opposition protests in 2012 against Mr Putin over allegations that polling had been fraudulent