Conor McGregor in police custody in New York
Irish martial artist Conor McGregor is in police custody in New York.
A New York Police Department spokesman said charges would be filed later.
An Ultimate Fighting Championship (UFC) statement said a vehicle that contained a number of athletes competing at an event taking place this weekend had been "vandalised".
It added: "The organisation deems today's (Thursday) disruption completely unacceptable and is currently working on the consequences that will follow."
The alleged trouble erupted at a UFC media day.
Beforehand, McGregor, 29, had been stripped of his UFC lightweight title.
He has not fought in the UFC for more than a year but lost to former professional boxer Floyd Mayweather in a money-spinning bout last August.
McGregor has been a flamboyant presence on the sporting scene and divided opinion in Ireland.
An NYPD spokesman confirmed the Irishman was in police custody.
He added: "No formal charges have been filed yet but they will be filed after 7 o'clock (12 pm BST)."
The incident happened at the Barclays Centre in the Brooklyn borough of New York City.
People had gathered to promote a competition headlined by Khabib Nurmagomedov and Max Holloway.
The UFC added: "NYPD was immediately alerted and is currently in the process of investigating the situation.
"UFC is working very closely with the New York Athletic Commission, Barclays Centre security and law enforcement authorities.
"All parties are working together to ensure the highest standard of proper safety precautions are implemented to protect the fans and athletes."