The head of the Olympic Council of Ireland (OCI) has stepped aside from his role after being arrested in Brazil as part of an investigation into the alleged illegal sale of tickets for the Rio games.
Patrick Hickey, 71, was detained at a hotel in the Barra da Tijuca area of the city and underwent cardiac tests in hospital amid concerns for his health.
The OCI revealed he had stood down temporarily from the top sports job he has held for 28 years.
Mr Hickey, from Dublin, has also stepped aside from his positions as the International Olympic Committee member in Ireland, president of the European Olympic Committees and vice president of the Association of National Olympic Committees.
Rio's Civil Police confirmed officers had detained Mr Hickey.
The OCI denied he had attempted to avoid arrest and said he was being treated in hospital as a precaution.
The organisation said: "In light of this morning's developments and his ill health, Mr Hickey has taken the decision to step aside temporarily as president of the OCI and all other Olympic functions (IOC member in Ireland, EOC president, ANOC vice president) until this matter is fully resolved.
"Mr Hickey will of course continue to co-operate and assist with all ongoing inquiries."
Mark Adams, International Olympic Committee director of communications, said the organisation would co-operate fully with police.
"What I can tell you is that it involves 1,000 tickets of the six million tickets (for the games)," he said.
"We have full confidence in the system here and we believe here, as with everywhere, that everyone is innocent until any guilt can be proved or, in fact, any charges have been brought."
The investigation centres on the sale of more than 800 tickets to the Games, including more than 20 which police said were intended for use by Ireland's Olympics officials.
Irishman Kevin Mallon, a director of THG Sports, which specialises in corporate and sports hospitality and is owned by Ipswich Town FC owner Marcus Evans, has been in custody in Brazil since August 5 over the affair.
Mr Mallon was arrested after police seized Olympic tickets in a Rio hotel, some of which were part of the Irish allocation and some for the opening and closing ceremonies and the football final.
Ireland's Sports Minister Shane Ross cut short his trip to the Games to hold crisis talks over the affair in Dublin with Attorney General Maire Whelan, colleagues and officials.
Mr Ross said: "Given the seriousness of this matter, and in the interests of taking swift and decisive action, I will be returning to Dublin as quickly as possible."
Mr Hickey is being supported by Irish diplomats in Brazil.
A doctor assessed the 71-year-old after police called to a hotel where he was pictured at the door of a room in a dressing gown.
He advised Mr Hickey to undergo tests in hospital before any attempts were made to question him.
The authorised ticket reseller (ATR) contracted by the OCI is Dublin-based PRO10 Sports Management.
The company said it had legitimate customers for its tickets and Mr Mallon was acting as a collection agent for them in Rio, meeting clients.
THG and PRO10 have denied any wrongdoing over ticket sales.