The head of France's network security agency said it was the biggest-ever hacker attack against the government.
France holds the rotating leadership of the G-20 this year and is hosting a series of meetings aimed at improving relations among the world's top economies, including the United States and China.
Other attempts to hack computers at the presidential palace, the Foreign Ministry and other ministries with information about the G-20 failed, according to the National Security Agency for Information Systems.
Those behind the attacks were after information about French financial and economic policies, said Budget Minister Francois Baroin.
Baroin said it's too early to say who was behind the attacks on Finance Ministry e-mail accounts and servers, but the authorities think they came "probably from outside" France.
"It was the information about the G-20 that interested the hackers," Baroin said in an interview on radio station Europe-1.
Officials at the Finance Ministry would not elaborate on Baroin's comments.
Network security agency chief Patrick Pailloux said "sensitive" information had been obtained in the attacks which probably began in November-December, carried out by "a number of professional, determined and persistent hackers."
Speaking on radio station France-Info, Pailloux said it was not the first time government computers had been attacked, but that "it's the first time that it has reached such proportions."
Pailloux said the attack took place late last year and the intruders infiltrated around 150 computers, out of a total 170,000 computers in France's Finance Ministry. - AP