Police detained the treasurer of Brazil's governing party in a wide-ranging investigation into corruption at state-run oil giant Petrobras.
The Workers' Party said later he had asked to resign from the post.
Joao Vaccari Neto was detained in Sao Paulo as he was heading out for an early morning jog, police told a news conference in the southern city of Curitiba, where the investigation is being led.
An arrest warrant also was issued for Mr Vaccari's sister-in-law, and his wife was questioned in connection with a series of unidentified deposits in her account that investigators suspect might be related to a bribery scheme at Petrobras.
Mr Vaccari has maintained his innocence, telling a congressional panel last week that all the donations made to his party during last year's election campaign were perfectly legal.
Workers' Party president Rui Falcao said that "for practical and legal matters Vaccari asked to step down as the party's treasurer".
But he also called Mr Vaccari's arrest unnecessary and expressed confidence in his innocence.
"We reaffirm our confidence in Joao Vaccari's innocence not only because of his conduct as treasurer, but also because in a democracy everyone is innocent until proven guilty," he said.
"The party is confident that in the end truth will prevail."
More than 100 people have been charged in connection with the Petrobras case, and more than 50 politicians are under investigation.
Among them are two former chiefs of staff to President Dilma Rousseff, but Mr Vaccari is the seen as closest figure to the president yet implicated in the scandal.
Ms Rousseff herself, who served as chairwoman of Petrobras' board during several years, is not implicated.
Mr Vaccari was detained "preventatively" and would be taken to Curitiba to testify, investigators said.
He was charged last month with corruption and money laundering in connection with the scheme, which prosecutors have described as the biggest uncovered in Brazil.
According to prosecutors, big construction and engineering firms paid at least 800 million US dollars (£540 million) in bribes and other funds in return for inflated contracts with Petrobras, which is Brazil's biggest company.
Part of that money allegedly went to the Workers Party and other parties for political campaigns.
Yesterday, police officer Igor Romario de Paula said officials had been collecting evidence against Mr Vaccari "for a long time".
He said the judge in the case, Sergio Moro, decided to order Mr Vaccari's detention due to his "criminal habits, recidivism, a clear danger to public order and because of the kind of projection he has, the influence of the job he has today and the possibility of his fleeing".
The newspaper Folha de S Paulo quoted Mr Moro as suggesting the arrest order was prompted by worries that Mr Vaccari, who is still treasurer of Ms Rousseff's party, might be able to influence the investigation.
Brazilian law sets no limit to how long those under preventative arrest can be detained.