Animal rights activists said they freed 9,000 pheasants as part of a campaign to “dismantle the shooting industry, farm by farm, shoot by shoot”.
The Animal Liberation Front (ALF) said it targeted Heath Hatcheries in Suffolk over the Easter weekend.
Suffolk Police said internal fencing, netting and a water pipe were cut, enabling the birds to escape.
Countryside Alliance said it was “completely unacceptable” and many birds would die in the wild.
The BBC has attempted to contact Heath Hatcheries.
ALF said on Facebook: “With game farms being targeted across the country, many shoots may well go out of business this year.”
The group said a clear path was made and grain was put down so the birds headed towards the forest and away from the road.
It said it surveyed the 45 breeding pens and removed a panel from each, “herded the pheasants out into the corridor then further on and out of the farm directly into a wooded area”.
Police said the attack happened overnight on Friday, between 21:00 BST and 07:00 on Saturday, at the site off Brandon Road, Mildenhall.
Investigators have appealed for witnesses.
Tim Bonner, the chief executive of Countryside Alliance, said: “This latest attack on a game farm by animal rights extremists is completely unacceptable.
“By releasing birds without making provision for them to adapt to the wild many of them will die as a result of these thoughtless and ignorant acts.”
He said game farmers “should not have to live in fear” and the attacks “must be investigated and the perpetrators brought to justice”.