The Justice Department is set to announce a major crackdown on cyber-crime that will include arrests, subpoenas and property seizures of alleged e-mail spammers and online scam artists, according to law enforcement and industry sources.

Attorney General John D. Ashcroft has scheduled a news conference for tomorrow , with some of the more than 100 enforcement actions continuing through tonight.

More than half the cases focus on online scams, especially "phishing," an increasingly common tactic in which computer users receive e-mails that look as if they are from banks or other legitimate businesses but are used to induce people to provide credit card numbers or other personal information.

Other cases involve the continuing onslaught of traditional spam, which can include pornography, fraudulent diet schemes or bogus body-organ enhancement drugs. Spam also has become a primary delivery system for viruses that can either disable a machine, expose private data or turn a computer into a "zombie" that can be controlled by a hacker to launch more spam.

Because the enforcement actions are ongoing, FBI officials declined to provide details.

"It's a large number of cases," said H. Robert Wientzen, head of the Direct Marketing Association (DMA), an organization that has put up $500,000 to assist law enforcement in tracking down the most notorious spammers.

The FBI, with help from the DMA, launched Operation Slam Spam a year ago, with technical operations at a field office in Pennsylvania. Wientzen said his organization provided financial and technical help.

The actions to be announced Thursday will be the first fruits of the effort, though it is unclear how many of the cases involve formal charges or indictments.

But Wientzen said he expects a second round of actions in the fall.

Spam, which accounts for as much as 70 percent of all e-mail traffic in the United States, has by some estimates cost businesses and consumers as much as $10 billion a year.

Phishing, a newer phenomenon, has zoomed up the charts of Internet crime. In the past year, transactions carried out after phishing scams cost banks and credit card companies an estimated $1.2 billion.

In May of this year alone, according to the Anti-Phishing Working Group, an industry consortium, more than 1,100 new scams were launched.

According to the research firm Gartner Inc., an estimated 57 million adults in the United States had received a phishing e-mail as of May.

Usually, the bogus e-mail looks as if it came from a bank, payment service or vendor such as eBay, claiming that account information, passwords or credit card numbers need to be verified. Often, they threaten to discontinue service if the information is not provided.

Users are directed to a phony Web site designed to look official, where they can enter the information requested. Computer experts advise consumers never to respond to such solicitations.

Staff writer Dan Eggen contributed to this report.

Attorney General John Ashcroft, left, is to discuss the crackdown tomorrow. H. Robert Wientzen heads a marketing group that has assisted law enforcement.