"It was the biggest home party so far," the 66-year-old said after her unexpected show in Stockholm's Globen Arena.
Schyman explained that the star had got in touch earlier in the day to discuss his thoughts on feminism.
"He phoned and was pretty well-informed, wanted to meet and support us in the election," she explained.
Williams introduced Schyman ahead of his hit song "Happy", calling on the audience to vote for the Feminist Initiative in Sunday's elections.
"So are you guys ready for Sweden to be on the news all over the world tomorrow?" Williams asked his audience.
While the move dominated headlines on Sunday morning, the party has had to withstand some criticism for acceding to the wishes of the controversial star.
Williams' feminist credentials have been called into question due to a song he wrote last year for Robin Thicke. The song "Blurred Lines" includes the line "I'll give you something big enough to tear your ass in two".
This was however nothing Schyman discussed with the star before accepting his offer to share his limelight.
"It is not as if we support the songs he does, it was instead he who wanted to support us, and that I thought was very nice."
The party took to Twitter to defend itself.
"Respectfully, those who criticize the evening's little coup: @gudschy (Gudrun Schyman) spreads our politics everywhere she goes. She does not shy from taking space."