A Brazilian reporter entered the athletics stadium for the Rio Olympics without any accreditation and spent two hours there before being noticed by security, he said on Friday.
Luiz Ernesto Magalhaes of O Globo newspaper wrote: "I can say that it was easy to approach places that are supposed to be very well guarded, like electrical generators."
The first Games in South America begin with the opening ceremony on August 5 and run until August 21 with around 85,000 security personnel due to be on duty.
Unlike most previous Olympics, the opening ceremony is being held in the famous Maracana football stadium rather than the smaller athletics arena, which is still one of the major venues for the Games.
Last week Brazilian federal police arrested 10 people suspected of plotting a terrorist attack at the Games.
The suspects, all of whom were Brazilians, were members of an online group calling itself the Defenders of Sharia.
Brazil's Justice Minister Alexandre de Moraes said they were an "amateur" and "disorganized" terror cell.
He added that the suspects had celebrated attacks claimed by Islamic State in Europe and the Middle East and had declared Brazil a potential target despite the country not being part of the coalition battling the terrorist group in Iraq and Syria.