The head of the Olympic Council of Ireland (OCI), Patrick Hickey, has been arrested in Brazil as part of an investigation into the alleged illegal sale of tickets for the Rio games.
The city's Civil Police confirmed officers had detained the sports chief more than a week on from the arrest of another Irish man in connection with the same inquiry.
It is understood Mr Hickey was arrested in a hotel in the Barra da Tijuca neighbourhood and there were unconfirmed reports that he was taken to hospital after complaining of feeling unwell.
Irish man Kevin Mallon, a director of THG Sports, which specialises in corporate and sports hospitality, has been in custody in Brazil since August 5 over the affair.
Mallon was arrested after police seized about 1,000 tickets for the Olympics in a Rio hotel, some of which were part of the Irish allocation.
PRO10, the OCI's authorised ticket reseller for the games, said it had legitimate customers for its tickets and Mallon was acting as a collection agent for them in Rio, meeting clients.
THG and PRO10 have denied any wrongdoing over ticket sales.
Mr Hickey had previously denied any wrongdoing in the OCI's role in securing or using an authorised ticket reseller for the games.
Irish consular officials in Brazil were aware of Mr Hickey's arrest.
A spokesman for the Department of Foreign Affairs in Dublin said: "We are aware of a case and stand ready to provide consular assistance if requested."
A spokesman for the OCI said he was aware of the reports in Brazilian media and awaiting official confirmation.
"We are still trying to get some clarity on the situation. When we do that we will release a statement through our normal channels," he said.
Shane Ross, Ireland's Sports Minister, flew into Rio on Sunday and had showdown talks with Mr Hickey about the ticketing affair.
After the meeting, Mr Ross said he was stunned by the OCI's attitude and its refusal to allow an independent figure on the internal inquiry which is probing the sale of tickets allocated to Ireland.