In a statement released after Thursday's meeting, which was held virtually and chaired by president Aleksander Ceferin, Uefa said it and its 55 member associations "stand as one".
"We unanimously and unequivocally reject Fifa's proposal to transfer ownership interests in the World Cup and other Fifa competitions to private investors," it said.
On Wednesday, Uefa had accused Fifa of using football "to enrich themselves and their friends".
Its statement on Thursday was even more damning, calling it "irresponsible and indefensible that a proposal of such significance for football was conceived in secret" and "without any meaningful consultation" with those who steward the game.
It added: "This is not merely a profound failure of leadership, but an abdication of Fifa's duty as the custodian of world football.
"National associations around the world are now presented with an ultimatum: accept the irreversible capture of football's greatest competitions or bear the consequences.
"This is not a 'democratic decision', but governance by intimidation - an act of coercion unworthy of an institution entrusted with the stewardship of the global game."
While Uefa only accounts for a quarter of Fifa's membership, it does include most of the world's most successful teams.
Six of the eight quarter-finalists - including winners Spain - at this summer's World Cup were European. In the 2022 and 2018 editions, the figures were five and six respectively.
Of the 23 World Cups to have taken place to date, 13 have been won by European countries. The other 10 have been won by South American sides.
Uefa vice-president Laura McAllister said Fifa's proposals posed a "genuine existential threat to the game".
The former Wales international told BBC Radio Wales she hoped boycotts would not happen, adding: "I feel optimistic they won't, because the reality is the powerhouse of world football is Europe."
Speaking to Radio 4's PM programme, McAllister said there was "real unity" among Uefa members and an "easy consensus over the need to take the strongest possible action given the existential threat that the Fifa proposal poses to our sport".
She added: "The reality is we had no choice other than to issue a threat commensurate with this dreadful, catastrophic proposal that has been rushed through with no due diligence, no proper governance and no consultation."