UFC, the US-based mixed martial arts juggernaut, said that more than 20 top celebrities from the worlds of sports and entertainment have piled in as investors.

Among the league's new investors are tennis star sisters Serena and Venus Williams, as well as Maria Sharapova; actors Sylvester Stallone and Ben Affleck, and Canadian singer Abel "The Weeknd" Tesfaye, UFC said in a statement.

Money reportedly has been pouring in for the fast-growing UFC, whose bloody cage-fights were once equated with cockfights.

UFC produces more than 40 live events a year that are broadcast in over 156 countries and territories.

Contacted by AFP, Ultimate Fighting Championship declined to give details about the amount of each new investment or the size of the share holding.

The batch of investments came less than three months after UFC was bought by a group led by US talent agency William Morris Endeavor-International Marketing Group.

UFC did not disclose the price, but US media said it was $4 billion.

Involved in the WME-IMG acquisition in July were Silver Lake Partners, Kohlberg Kravis Roberts (KKR), MSD Partners and MSD Capital. The latter two are investment vehicles for entrepreneur Michael Dell.

According to a person with knowledge of the situation, WME-IMG remains the majority stakeholder in the organization.

WME-IMG bought the organization from brothers Lorenzo and Frank Fertitta, who had bought it for $2 million in 2001.

UFC does not publish its financial details, but Lorenzo Fertitta said last year that it had $600 million in revenues in 2015.

UFC fighters battle in an Octagon, an eight-sided 750-square-foot (70-square-meter) cage.

The fighters, gloved and barefoot, are allowed to combine techniques from different forms of martial arts, but rules try to limit serious injury.

After years of being banned, mixed martial arts fights are allowed in the state of New York. The UFC's first match in Madison Square Garden in New York City is set for November 12.