An estimated 300-400 people congregated at Mynttorget in Gamla Stan to voice their disaproval of the new law, which will enable the National Defence Radio Establishment (FRA) to monitor all email and phone calls crossing Sweden's borders.
The protest action organized by the newly formed Banana Republic network brought together leading figures from the opposition Green and Left parties, as well as leaders from the youth organizations of the majority of parties represented in parliament.
Speaking to The Local, Green Party spokeswoman Maria Wetterstrand said that her party was far more scathing in its criticism of the law than potential coalition partners Mona Sahlin and her Social Democrat party.
"Mona has said she will tear up the law should they win the next election. But they will probably replace it with a similar one. We will make sure that doesn't happen," she said.
Wetterstrand also took the opportunity to stock up on some fruit as the event organizers handed out bananas among the crowd.
"They did give me one," said Wetterstrand.
"But I don't think Sweden is on the verge of becoming a banana republic."
As the sun beat down and a samba band prepared to play, Pirate Party leader Rickard Falkvinge addressed the crowd as "my fellow countrymen from the Banana Republic."
The demonstration was the first of two anti-FRA law events scheduled to be held in the capital on Thursday.
During the evening, regulars from the city centre's champagne set are planning to join forces at Café Opera to "party against mass surveillance". At the bar guests will be able to order cocktails named after both the current and previous defence ministers, Mikael Odenberg and Sten Tolgfors, both of whom have argued in favour of the law.
Paul O'Mahony & Faisal Enayat Khan