Ukraine president Volodymyr Zelensky says heavyweight champion Oleksandr Usyk has been released after being detained at an airport in Poland.
A video posted on social media showed Usyk being led away in handcuffs by uniformed officials.
"I talked on the phone with Oleksandr Usyk when he was detained," Zelensky wrote on Telegram.
"I was outraged by this attitude towards our citizen and champion.
"I've instructed the Minister of Foreign Affairs of Ukraine Andrii Sybiha and the Minister of Internal Affairs of Ukraine Ihor Klymenko to immediately find out all the details of the incident at the Krakow airport.
"As soon as I was informed that everything is fine, our champion was released and no-one is detaining him anymore."
WBC, WBA and WBO heavyweight champion Usyk is expected to be in London this week to watch Daniel Dubois defend his IBF title against Anthony Joshua at Wembley.
"We immediately responded to the detention of Oleksandr Usyk in Krakow and facilitated his release," Sybiha wrote on X, external.
"I was informed of the details by our Consul General. We consider such actions to be disproportionate and unacceptable towards our champion and will send a note to the Polish side."
Usyk's promoter Alex Krassyuk said on Instagram the incident was a "misunderstanding".
The boxer also said it was a "misunderstanding", but "was quickly resolved".
"Thanks to all who got concerned," Usyk wrote on X., external
"Thanks to Ukrainian diplomats for the efficient support. And respect to Polish Police for conducting their obligations with no regards to height, weight, reach and regalia."
It is not clear why Usyk was detained.
BBC Sport has approached Usyk's manager and Krassyuk for comment.
Usyk has not fought since 19 May when he beat Tyson Fury to become boxing's first four-belt undisputed heavyweight champion.
The Ukrainian vacated the IBF title in June after opting not to face his mandatory challenger in favour of agreeing a rematch with Fury, which is scheduled for December 21.
Usyk was also the undisputed champion at cruiserweight and is unbeaten in 22 fights, winning 14 inside the distance.