Undisputed heavyweight world champion Oleksandr Usyk has been released after detention by law-enforcement officers at Poland's Krakow airport, Ukraine's President Volodymyr Zelenskiy said on Wednesday.

"I was outraged by this attitude towards our citizen and champion," Zelenskiy said on the Telegram messaging app.

"Our champion was released and no one is detaining him anymore."

It was not immediately clear why the 37-year-old Usyk was detained. The WBC, WBO and WBA champion, who also won gold at the 2012 London Olympics, has been a national hero aiding Kyiv's war efforts.

Poland's foreign and interior ministries did not immediately reply to Reuters' requests for comments.

"Friends, everything is fine," Usyk said in an Instagram post. "There was a misunderstanding that was quickly resolved. Thank you to everyone who was concerned."

He added, "Respect to the Polish law enforcement officers who perform their duties regardless of height, weight, arm span, and titles."

Ukrainian Foreign Minister Andrii Sybiha said on X that his ministry will contact the Polish foreign ministry, as it considered the detention "disproportionate and unacceptable in relation to our champion."

Polish TVP Info, a television news channel run by state broadcaster TVP, published a social media video on its website showing Polish law-enforcement officers walking the handcuffed Usyk through what appeared to be an airport.

Reuters was not able to independently verify the video.

Usyk beat Tyson Fury in May to become undisputed heavyweight boxing world champion in a thrilling contest at the Kingdom Arena in Riyadh.

The 37-year-old Ukrainian is the first boxer to hold all four major heavyweight belts at the same time and the first undisputed champ since the end of Lennox Lewis' reign in April 2000.

Usyk's charity fund, Usyk Foundation, aids Kyiv's forces in the war that Russia launched with a full-scale invasion against Ukraine in 2022. It buys, among others, ambulances and delivers humanitarian aid to the front line.

Usyk's wife, Yekaterina Usyk, who posted a blurred photo of her husband surrounded by uniformed officers, said in an Instagram post in English that she was thankful her husband was free after a misunderstanding.