The bodies were discovered on the island of Olan, just off the southeast coast of Sweden, during archaeological digs studying the Migration Period in Scandinavian history about 400 years before the Viking Age.
The custom of the time was to burn the dead, the researchers said, so it was a shock when they unearthed intact remains.
"We have a massacre and everyone is still there," archaeologist and project manager Helena Victor of Lund University told The Local.
"In the way they've been killed it's clear it's not a domestic fight," she said. "These people were just in their houses. Slaughtered in their houses."
Five bodies were discovered in one hut alone last week, the archaeologists said, and the ongoing dig is uncovering more.
"We have just opened very small trenches, about 1 percent of the site, and already found 10 people in different places," Victor said. "I'm expecting a couple of hundred."
The motive for the massacre remains unknown, researchers said.
"We have no trace of the attackers," Victor said. "We're not sure if they attacked during the day or night or how it came to be. We don't know if they came by water or land. We don't know if they were Swedes or Finns or Danes or anything else. That's the big question."