Fifa's hailing of its Club World Cup has been labelled "nothing more than a fiction" by the president of leading players' global union Fifpro.
In a scathing statement, Sergio Marchi appeared to compare the world governing body's president Gianni Infantino to the Roman emperor Nero.
And he also claimed Fifa "chose to continue increasing its revenue at the expense of the players' bodies and health".
It comes after BBC Sport learned that Fifpro was not invited to a key meeting on player welfare that Infantino held on the eve of the Club World Cup final with representatives from other unions.
Last year, amid a backlash over the expansion of the Club World Cup, Fifpro filed a legal complaint against Fifa, claiming it had abused its role under European competition law by adding more pressure to the fixture schedule.
Fifa has denied the claims, and at the weekend Infantino called the tournament "the most successful club competition in the world".
But tensions have now intensified, with Fifpro president Marchi insisting that while the Club World Cup "generated enthusiasm among numerous fans and allowed some of the world's leading figures to be seen in a single tournament... this competition hides a dangerous disconnect with the true reality faced by the majority of footballers around the world".
Marchi added: "What was presented as a global celebration of football was nothing more than a fiction created by Fifa, promoted by its president, without dialogue, sensitivity, and respect for those who sustain the game with their daily efforts.
"A grandiloquent staging inevitably reminiscent of the 'bread and circuses' of Nero's Rome, entertainment for the masses while behind the scenes inequality, precariousness, and the lack of protection for the true protagonists deepen."
Fifth Roman emperor Nero threw spectacular games and events which made him popular with ordinary people, but is better remembered for his brutality and cruelty.
Fifa has previously said all its revenues for the tournament will be redistributed back to the club game for participating and non-participating teams.
At the weekend, FIFA said there was a "consensus" with player unions that there should be at least 72 hours of rest between matches, and players should have a rest period of at least 21 days at the end of each season. However, that is a week less than Fifpro had been asking for.
Fifpro - which represents 66,000 men's and women's players around the world - has previously expressed concern about the extreme heat faced by players in the US during the tournament, demanding more flexibility from Fifa on kick-off times and protocols. Infantino has admitted that the heat was "an issue".
But Marchi said, "the tournament was held under unacceptable conditions... that put the players' physical integrity at risk".
He added: "This situation must not only be denounced, but must be firmly warned against. What happened cannot be repeated under any circumstances at next year's Fifa World Cup.
"We have been warning about the overcrowded schedule, the lack of physical and mental rest for players, and the lack of dialogue on the part of Fifa. This way of organising tournaments, without listening to the federation that represents the world's professional footballers' associations, is unilateral, authoritarian, and based solely on a logic of economic profitability, not human sustainability.
"We cannot continue to play with the health of players to fuel a marketing machine. There is no spectacle possible if the voices of the protagonists are silenced."
Fifa has been approached for comment.
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