Police in Brazil investigating illegal Olympic ticket sales have arrested the head of the European Olympic Committees, Irishman Patrick Hickey.
Mr Hickey, 71, is suspected of being involved in a scheme to sell tickets for higher than their face value.
He was taken to hospital as a precaution, and will probably stay the night there, police said.
The Olympic Council of Ireland (OCI) said he had stood down temporarily from his roles after being arrested.
Brazilian police said that when officers went to Mr Hickey's hotel room on Wednesday morning, they found his wife there with his Olympic credentials.
Mrs Hickey told them her husband had left Rio de Janeiro for Ireland at the weekend, they said.
With the help of the hotel they began a search and found Mr Hickey in the room next door, they added.
While police said they had reason to believe he was trying to escape, a video apparently of the arrest on the ESPN Brazil sports news website shows Mr Hickey answering the door naked to officers.
Police said the arrest was related to that of fellow Irishman Kevin James Mallon on the day of the Olympic opening ceremony.
More than 700 tickets organised in envelopes clearly marked for sale were found in a safe with Mr Mallon, Detective Ricardo Barbosa told the BBC.
Police estimate the potential profits of the alleged scheme at 10m reals (£2.4m; $3.1m).
The International Olympic Committee (IOC) said it would co-operate with any police investigation.
Irish broadcaster RTE reports that Mr Hickey faces three potential charges: facilitating ticket touting, forming a cartel, and "ambush" or illicit marketing.
In an interview with RTE last week, Mr Hickey denied any wrongdoing in relation to ticket sales.
A spokesman for Ireland's Department of Foreign Affairs and Trade told the BBC: "We are aware of an Irish citizen having been arrested in Rio. The department, through its consulate in Rio, is following up and stands ready to provide all appropriate consular assistance."
Sports minister Shane Ross, who is in Rio and who met Mr Hickey at the weekend to discuss the ticket sales allegations, tweeted simply: "Shell shock here in Rio."
He later said he was returning to Dublin as quickly as possible for crisis talks.
Mr Hickey is president of the European Olympic Committees, the body which gathers the continent's 50 national Olympic committees together.
His passport and Olympic ID were seized and were later displayed to photographers at the press conference.
His arrest is the biggest move yet by police investigating the selling of tickets on the black market.
Who's who in Irish Olympic tickets controversy?
- Patrick Hickey - president of both the European Olympic Committees and the Olympic Council of Ireland (OCI)
- OCI - the state-funded body responsible for selecting, training and preparing Irish athletes to take part in the summer and winter Olympics
- Kevin James Mallon - Director of THG Sports
- THG Sports - A corporate and sports hospitality company based in London.
- Pro 10 Management - An Irish sports management company based in Lucan, County Dublin, which was appointed by the OCI as its Authorised Ticket Re-seller (ATR) in Ireland.
Mr Mallon is the director of THG Sports, a British sports hospitality company alleged to have resold tickets for Rio 2016 illegally.
Police suspect the tickets found with him when he was arrested on 5 August were to be sold at prices of up to £6,000 ($7,800) each.
THG Sports said Mr Mallon had not sold or sought to sell tickets but had been holding them for collection by clients of the authorised Irish ticket reseller, Pro 10 Management.
But Detective Barbosa told the BBC that investigators believed "Pro 10 was created as a means for acquiring tickets to be resold by THG" as the Irish Olympic Committee had wanted THG to sell tickets for Rio 2016, he alleged.
THG Sports is owned by Marcus Evans Group, which also owns Ipswich Town Football Club.
Marcus Evans was one of four names on a list of arrest warrants issued by a Brazilian judge on Monday in connection with the alleged reselling of Rio 2016 tickets.
None of those four people is thought to be in Brazil and police there have said they will pursue the matter with Interpol.
A further three arrest warrants were issued on Wednesday, Brazilian police said, for directors of Pro 10 Management.
The OCI has said it will launch an inquiry into the ticket sale controversy but has been criticised for rejecting a call by the Irish sports minister to bring in independent investigators.
Last week, the OCI wrote on its website there was "absolutely no suggestion of misconduct or impropriety" by the OCI or any of its staff.