Memo to U.S. workers: Your days of online shopping or other leisure Web-surfing at your desks may be ending.

Across the country, thousands of large and small firms -- and government agencies -- are installing special software that can block access to individual Web sites from employees' computers. And while many companies are only blocking the obvious suspects, such as sites featuring pornography, gambling or hate speech, some firms are taking matters a step further.

At financial powerhouse Merrill Lynch & Co., grumbling employees said they are prevented from getting to online auctioneer eBay and other e-commerce sites. Even widely used search engines are blocked for some workers.

"They tell you they trust you with a million-dollar account, or with someone's life savings, but you can't be trusted to look at the Internet," said one Merrill worker. A company spokesman acknowledged that Merrill uses the software but refused to provide further details.

As Internet use has mushroomed over the past few years, employers have increased efforts to monitor how their employees use e-mail and the Internet, concerned about everything from worker productivity to employees inadvertently bogging down networks or leaving systems open to viruses.

According to an FBI survey last year, 78 percent of companies found that some of their employees had abused their Internet privileges, such as by downloading pornography or pirated software. An Internet study by the research firm IDC estimates that 30 to 40 percent of Internet surfing between 9 a.m. and 5 p.m. is not work related.

But monitoring Web use has put firms in the position of spying on their employees, which can cause friction and has mobilized privacy and workers-rights advocates. So companies are increasingly turning to a new generation of blocking software that can be highly customized.

"That's the secret to this space," said Peter Kuper, who follows the industry for investment firm SG Cowan Securities Corp. "Companies don't want to be too Big Brother-ish, but want to prevent everything from porn to malicious code." Kuper said companies are especially concerned about legal liability if their employees use the Web improperly, such as by viewing sexually explicit content that others around them can see.

In addition to preventing access to an entire class of Web sites, such as those devoted to pornography or hacking, the software can allow companies to permit other types of sites, such as entertainment or chat rooms, to be seen only after 5 p.m. or for only a certain amount of time each day. And the company can set the controls to govern use by individuals or departments.

San Diego-based Websense Inc., one of the leading vendors of what is being called employee Internet management (EIM) software, practices what it sells.

Each company employee is only allowed to look at stock-trading, shopping, sports and entertainment sites for a total of 45 minutes per day, said Andrew Meyer, the vice president of marketing at Websense.

When the firm delivers its software, it contains a database of 4.2 million Web sites, broken into dozens of categories, which the client company can choose to block, allow or modify by adding or deleting sites.

Websense has 18,000 customers worldwide, including 282 Fortune 500 companies, from General Motors to Coca-Cola and Marriott Resorts. Sales grew 57 percent last year over 2001. A chief competitor, SurfControl, whose global headquarters are in England, also provides e-mail filtering. It was the leader in Web-filtering software sales in 2001 and has seen similar gains.

Overall, analysts expect Web-filtering to be an $800 million industry by 2006.

Some companies, such as Merrill Lynch, are continuing to monitor Web and e-mail use even as they block certain sites automatically.

But privacy advocates, while uncomfortable with restrictions on Internet behavior, are relieved that many companies are moving away from monitoring and toward blocking. And they recognize that workplace computers are corporate property.

"To think that Fortune 500 companies can run their Web site access completely on an honor system is a bit naive. There's too many downsides," said Lewis Maltby, president of the New Jersey-based National Workrights Institute, an employee advocacy organization.

He called monitoring of Web visits "a privacy nightmare," since people often look to the Internet for help with medical, family or financial problems.

Susan Getgood, marketing vice president at SurfControl, said that companies whose Web sites are listed in categories to be blocked unless modified have not complained.

"Companies understand it's just a management tool," she said.

Kevin Purseglove, spokesman for online auction giant eBay, agreed.

"We don't worry about it," Purseglove said. "We understand employers who want their employees to attend to their jobs."

Ironically, though, Purseglove said that when eBay first launched its site employees were not allowed to bid on auctions during business hours.

But they complained, arguing that the policy put them at a disadvantage relative to workers around the country. So the company loosened the rules.

"Now we leave it up to managers to be sure people are performing their duties satisfactorily," he said.