Brazilian police arrested top International Olympic Committee member Patrick Hickey as part of an operation against black market Rio Olympics tickets.
The 71-year-old head of the European Olympic Committees and Ireland's national committee, was detained at dawn at a luxury Rio hotel where he is staying with other IOC members, police said.
According to Brazilian media, Hickey tried to escape when the operation was launched.
Following the arrest, the Olympic Council of Ireland (OCI) released a statement which said that Hickey had 'temporarily' stood down as head of the national committee.
Pictures shown by ESPN Brazil showed Hickey speaking to police in a bath robe in his room.
Hickey was taken away from the hotel in an ambulance at about 7.00am and was expected to spend the night in hospital, IOC spokesman Mark Adams and police said.
He has been accused of ticket scalping, ambush marketing and conspiracy. Prosecutors have estimated the profits from the black market sales at USD 3 million dollars.
"We will be fully cooperating with any police investigation," Adams told a press conference.
"Mr Hickey is entitled for the world and everyone to believe he is innocent until proven guilty," he added. "Let's not jump the gun on this one."
Brazilian police have launched a widening investigation after seizing more than 1,000 premier tickets for Games events, including the opening ceremony.
Tickets with a face value of about $1,000 dollars were sold for more than $8,000.
Some of the tickets had the Olympic Council of Ireland name on them. The OCI would only say that it was "seeking total clarity on the situation" before commenting.
Kevin James Mallon, a director of THG Sports, was arrested in Rio on August 5 in possession of the tickets.
THG Sports was licensed to resell London 2012 and Sochi 2014 Olympic tickets. Hickey's son Stephen used to work at THG.
Another THG staffer was arrested in Brazil in 2014 over the reselling of World Cup tickets.
Brazilian prosecutors issued warrants for Hickey and other figures including Marcus Evans, a British hospitality businessman and owner of English Championship football club Ipswich.
Echoes of FIFA arrests
Hickey has been a member of the IOC executive board since 2012. As head of the European body he has been instrumental in setting up the European Games which was first held in Baku, Azerbaijan, last year.
The arrest has echoes of a police raid on a FIFA hotel in Zurich last year in which seven officials of the football governing body were held on corruption charges. It will come as a blow to the IOC which has made anti-corruption efforts a priority since a bribery scandal around the 2002 Salt Lake City Winter Olympics.
Six IOC members were expelled after the scandal.
Hickey senior denied any connection to THG in an interview last week with Irish broadcaster RTE.
"There is no impropriety whatsoever from anyone within the OCI or myself in the dealing of tickets. I want to re-assure you of that 100 percent," Hickey said.
"I am fully expecting we, the OCI, will come out of it with a fully clear bill of health."
Brazilian police are also looking for an executive with Pro10 Sports Management, an Irish company based in Lucan which was appointed by the OCI as its authorised ticket re-seller in Ireland.
Pro10 said the tickets Mallon had in his possession were held for collection by Irish and other European customers who had purchased them at face value, plus a commission.
In a statement they called allegations against the company "utterly untrue."
"Pro10 is appalled that allegations are being made against the company without any attempt to ascertain the facts. The Brazilian authorities seem to be dealing with these extremely serious allegations through the media and no contact has been made with Pro10," it said.