"Of the Russian embassy's diplomatic staff, about one-third of them are not actually diplomats, they are in fact intelligence officers," Saepo's chief counter-espionage analyst Wilhelm Unge told reporters as the agency presented its annual security report.
"This is a very constant number, this is the way things look year after year," he said.
Russia's Foreign Intelligence Service SVR, military intelligence GRU and the Federal Security Service FSB were all present in Sweden, Unge said.
He said the modern-day spies were "highly educated, often a little younger than they used to be in the Soviet era, driven, determined, and socially competent. And they network."
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Unge's comments come amid concerns in Sweden over Russia's military resurgence.
Last October, a week-long search for a suspected Russian submarine in the waters off Stockholm was called off despite members of the public reporting numerous sightings of suspicious vessels.
The search in Stockholm's archipelago, involving battleships, minesweepers and helicopters, stirred up Swedes' memories of Cold War cat-and-mouse games with suspected Soviet submarines along Sweden's long, rugged coastline.
Together with a series of alleged airspace violations by Russian jets over the last year, it helped bolster public support for NATO membership in the non-aligned country.