He was among a record 4,002 people arrested across Russia last week, as large crowds took to the streets to demand the release of the opposition politician Alexei Navalny. Further protests have been called for Sunday, threatening to strain the system even further.
"We didn't sleep all night. One person always had to stand for space, so we took it in turns," Filip told me by phone on Wednesday from the back of the police van, which he described as an "ancient vehicle with metal bars all round".
Anti-corruption campaigner Alexei Navalny, fiercely critical of the Kremlin, was imprisoned on his recent return to Russia after recovering from an attempt to kill him with a nerve agent.
A judge had sentenced Filip to 10 days in custody on Monday for his part in the protest. But it took police a further two days to find space in city cells. By the time we spoke, his group had been waiting outside Moscow Detention Centre No 2 for 17 hours, fed only by volunteers who brought sandwiches to the van.
'Anyone could be next' Filip, a small-business owner, is not a fan or follower of Mr Navalny, but he is disturbed by how a fellow citizen is being treated.
"They're punishing him for nothing - which means anyone could be next, including me," he explained.
Few protesters we spoke to at Moscow's Pushkin Square last Saturday mentioned Mr Navalny's near-fatal poisoning, but all were shocked at how his flight home had been diverted so he could be detained at the border. He then faced a bizarre, makeshift court hearing in a police station.
Alexei Navalny: Russia's vociferous Putin critic One poll suggests as many as 42% in the Moscow crowd were moved to protest for the first time. The rallies were also unprecedented in their spread, covering towns and cities usually seen as politically "passive".
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