According to their lawyers, Dubovik and the others were entrapped by undercover agents.
"The increased security measures [around the World Cup] have led to innocent people getting in trouble, and dishonest law enforcement officials are using this to their advantage," lawyer Maxim Pashkov said.
"The atmosphere is such that they arrest first and figure out what
actually happened later."
Normally, pre-trial detention would be extended by two months rather than three, he said.
Nine anti-fascists have been arrested for allegedly plotting to set off bombs during the World Cup, accusations Human Rights Watch said were based on confessions extracted under torture.
On Tuesday, a Moscow court sentenced Roman Sushchenko, a Ukrainian journalist who was held during a 2016 trip to Russia, to 12 years for spying. Police have also reportedly been conducting raids against members of the Jehovah's Witnesses religious organisation, which was banned last year.
In the case of Dubovik, she and several friends had begun discussing politics on the messenger app Telegram, which Russia has tried to ban, following a failed "revolution" of mostly young protesters called by self-exiled nationalist Vyacheslav Maltsev in November.
They later began meeting in a McDonald's and going to protests in Moscow.
A newcomer to the group, Alexander Konstantinov, rented an office for the would-be activists and wrote a political manifesto calling for Putin to be tried by a "people's tribunal", according to his testimony.
Konstantinov was in fact gathering evidence against the young people.
Opposition activist Ildar Dadin, who was released from prison last year after Amnesty International declared him a prisoner of conscience, asked how football teams could participate in a World Cup "in a country that kills and imprisons innocent people".
- Telegraph Group Ltd