At least 33 civic groups nationwide — many of them anti-nuclear organizations — have been hit by a series of cyber-attacks that bombarded them with a total of 2.53 million emails since mid-September, group members and a lawyer representing them said Sunday.

The civic groups are considering filing a criminal complaint against the senders of the emails for obstruction of business, according to lawyer Yuichi Kaido, who represents the organizations, which includes Fukushima Genpatsu Kokusodan, a group formed by residents of the prefecture affected by the nuclear disaster at the No. 1 plant.

The first wave of emails arrived in September, flooding the 33 groups with over 2 million messages, the lawyer said. More messages were sent to two of the groups between late October and earlier this month.

While some of the emails contained hostile messages, the senders are believed to have used software called Tor to exploit computer systems that automatically send confirmation emails to people registering for online newsletters or making inquiries.

The perpetrators apparently obtained the email addresses of the civic groups and used them to register on websites that automatically return registration confirmations or reply messages.

Tor was also used to block the tracing of Internet activities and conceal email senders' locations.

A member of the Fukushima residents group said around 140,000 emails were received, thanking it for registering for online newsletters, wreaking havoc on the group's email activities.