The dining-entertainment venue, based on the Greek taverna featured in the ABBA-themed film Mamma Mia!, is the idea of one of the group's star Bjorn Ulvaeus, reported BBC.
He was joined for the opening night by Benny Andersson, Anni-Frid Lyngstad and Agnetha Faltskog, local news website thelocal.Se said.
"We are here to party," Ulvaeus told reporters.
The group were last seen in public together in 2008.
Fans and guests paid 110 pounds for a ticket to the opening night of Mamma Mia! The Party at the Tyrol restaurant in Grona Lund, Stockholm.
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The foursome split in 1982 and hasn't performed together since 1986. ABBA's music has enjoyed a huge boost thanks to the lucrative Mamma Mia! stage show, which opened in 1999 in London and later became a Broadway hit.
In 2008, the musical was adapted into a hit movie starring Meryl Streep and spawned its own best-selling soundtrack.
The premiere of the movie Mamma Mia! marked the last time ABBA's founding four gathered in public.