Elsewhere, police force open the rolling shutters of a popular cocktail bar and storm into the dimly-lit interior, where 36 people are drinking in the glow of neon bar signs. “Who's the owner?” barks a woman police officer. “Everyone line up with your papers in hand.”
Although Madrid's nightlife has been shuttered since the summer, and a midnight curfew has emptied the streets since October, such raids have become a regular feature in the city.
While many European nations have languished under a second lockdown, Spain has chosen to rely on restrictions, with bars and restaurants in Madrid operating with a 50-percent capacity limit and private gatherings limited to six people.
But in some places — as in other European countries — the party hasn't stopped, with people drinking, dancing and sweating together, up close and personal as if the pandemic never happened.
Some gather in bars for an after-hours lock-in, others have been invited to flats or houses rented for the weekend, or joined a rave-type event in a park or warehouse attracting hundreds of people, police say.
At most, masks are absent, capacity limits ignored and all other safety precautions checked at the door.
From lockdown to lock-ins
With the streets off-limits after midnight, house parties have become increasingly popular, Madrid police chief suprintendent Jose Luis Morcillo says.
“Although the immense majority are complying with the rules, there's a minority looking for alternative ways to have fun,” he told AFP.
“The average age is around 30, so they are no longer young, but they are irresponsible.”
Often events are organised by club owners whose venues are closed but who reach out to their guest lists. Secrecy is key and an entrance fee is “normal”.
“What's more scary than the situation we're living through with lockdowns, restrictions and fewer and fewer freedoms?” said one Halloween invitation with a 20-euro entrance fee and the venue to be confirmed on the day itself.
“Having 90 people in a flat implies risk.. people are very close together, dancing, not wearing masks, smoking and sometimes taking drugs,” Morcillo said.
Between late October, when a night curfew took effect, and mid-December, Madrid police have broken up 2,910 parties, charged 279 venues and penalised 7,816 people for not wearing a mask.
And the fines are not insignificant: €600 ($740) for just attending a party, €600 for not wearing a mask, and another €600 for smoking.
For organisers, the top penalty is €600,0000 — which has happened with two events in Madrid's industrial outskirts.
But most parties are in homes, police say.
A fine mess
There were nine in the flat, playing music, having a beer and a smoke on the balcony when they spotted the police car, sending everyone into a panic.
“When the police car appeared, three ran out. But one came back up with the police, so they knew at some point we were more than six so it was illegal,” said “Paloma”, an unemployed 29-year-old.
“The policeman said: 'There are two ways to do this, the good way or the bad way. Either you let me in or I get a warrant',” she said.
Eventually, they opened the door — but didn't get fined.
“Everybody was really nervous — they knew the police could have ramped up the fines, which would have been a nightmare for this nonsense.”
Few people talk openly about parties, and none want to use their real name.
“We had a party at our flat with about 15 people. That might seem selfish, but I don't see us changing something for the worse,” says “Oscar”, 29, who didn't want to be identified because his mum's name is on the lease.
And he's not worried about the police: he knows they can't enter without permission or a warrant, meaning “they can't fine you, because they can't count the people inside”.
'What about the metro?'
Like many in their twenties, he's unimpressed by the region's awareness campaign with huge billboards in the metro warning: “If you go to a party, the next stop could be the morgue.”
“You can't pretend that metro always being full of commuters is not a risk and that 15 of us being together in a flat is,” he scoffs.
Experts say it's impossible to know exactly where infections occur, though anecdotal evidence points to private gatherings where people drop their guard — and their masks.
“We don't have real data that tells us exactly what percentage of infections happens… at parties, in bars or restaurants or taking public transport,” says Salvador Macip, a medic and researcher at Britain's Leicester University.
Although the epidemiological situation had improved in November, Spain has seen a rise in coronavirus infections over the past few days that could worsen with Christmas looming, making safety measures all the more crucial.
By AFP's Hazel Ward