Ingrid Isgren, Sweden's deputy chief prosecutor, arrived in a people carrier shortly before 10am UK time. She posed for pictures on the steps of the Ecuadorian embassy but did not answer questions from the assembled media.
Isgren and a Swedish police inspector are present but the interview of Julian Assange is being conducted by an Ecuadorean prosecutor, who is putting forward questions they have already submitted.
The Swedish Prosecution Authority said that, providing Assange gives his consent, a sample of his DNA will also be taken.
The accusations against Assange stem back to time he spent in Sweden in 2010. Three of those offences have already passed the statute of limitations, but he can still be questioned about an allegation of rape.
The Australian has always protested his innocence, but refused to go to Sweden for questioning fearing he would be extradited to the United States for publishing classified documents online.
Sweden’s Director of Prosecution Marianne Ny, who has led the investigation, said she welcomed that fact “that the investigation can now move forward via an interview with the suspect”.
Assange’s lawyer Per Samuelsson told AFP news agency the questioning of his client is planned to last three days. He added that it was too early to know what would be made public.