The 40 year-old engineer was found in a flat in Botafogo suburb, Dona Mariana Street, by the Police of Bandeira Square Station (Praça da Bandeira).
Mario Santoro didn't resist the arrest and he said "I destroyed my own life".
Talking to a lawyer, he declared that he's innocent.
This early Sunday morning, Australian time (Saturday night in Rio) he was moved from the Police Station of Bandeira Square to the Rio de Janeiro Homicide Division headquarters, the division in charge of the case in Brazil.
Mario Marcelo Santoro is the main suspect in the murder of 38 year-old Brazilian business woman Cecilia Haddad, whose body was found in Sydney's Lane Cove river on the 29th April.
Brenno Carnevale, the Head Commissioner of Bandeira Square Police Station told Globo media that Mario Santoro was at the moment at the police station, where his profile was being assessed.
Brazilian police were hunting for Mario Santoro since Friday, after a Brazilian court has issued an arrest warrant for Santoro, who was considered a fugitive in both Australia and Brazil meaning he could be arrested on the spot in either country.
Haddad's body was found in Lane Cove River, in Sydney's north, on 29th April.
Santoro, Haddad's ex-boyfriend is considered to be the prime suspect in the murder investigation.
It's understood their relationship had ended weeks before her death.
The 40-year-old engineer is believed to have left Australia for Rio de Janeiro on the same weekend that Haddad's body was found.
The Homicide Division is leading the investigation in Rio de Janeiro since Haddad's family reported her death in early May.
News reports in Brazil today are even referring to the cause and manner of Cecilia Haddad's death: possibly strangled.
Yesterday, the Rio de Janeiro Homicide Division confirmed police had spoken to Haddad's relatives and had issued a warrant for Santoro's arrest.
“The Homicide Division began the investigation by listening to relatives of the victim, Cecília Müller Haddad, as well as providing expert analysis of some documents that the family of the victim Cecília Müller Haddad provided,” the statement said.
“At the request of the Homicide Division and favourable manifestation of the MP, the state court of Rio de Janeiro decreed, early yesterday evening, the arrest of the indicted Mário Marcelo Ferreira dos Santos Santoro.”
Since the warrant was issued police have scoured Santoro's home in Rio de Janeiro as well as the home of his parents in Copacabana and the homes of other relatives in Rio, to find him now in Botafogo suburb.
The Brazilian arrest warrant comes a week after Australian police and Interpol issued warrants for Santoro's arrest.
