Now that Web logs -- blogs, for short -- are a popular online pastime for millions of people, scammers are finding new ways to exploit them as vehicles for junk advertisements.

The Internet has even coined a term -- splog, a combination of spam and blog -- for a phenomenon that follows in the footsteps of rogue advertising such as spam e-mail, junk mail, junk faxes and adware. The new forms of spam can show up on blogs as fake comments posted by readers that actually have nothing to do with the subject at hand. Instead they are advertising pitches or attempts to get you to click on an unrelated Web site.

They also can be set up as bogus blogs; go looking for a blogger talking about, say, bathroom renovations, and you could wind up on a Web site that has a few random renovation-related words but that mainly tries to get you to click on links to advertisements.

For the most part, the ads are new pitches for old schemes -- gambling, porn -- and are posing enough of a customer nuisance that Internet giants such as Google and Yahoo are developing tools to clamp down on them.

Blogs are free and easy to set up, and until now, they have mostly been earnest forums for political and personal discourse. But their success is feeding the splog problem, because the more people look at blogs, the greater the potential audience for spammers. Some bloggers and search-engine users are calling on companies that help set up blogs to better police their systems.

"Yahoo and Google are the common carriers of the information age, and they have a reasonable responsibility . . . to prevent the illegal and inappropriate use of their services," said Scott Allen, an Austin-based online editor for About.com who also maintains a blog.

Last month, Blogger, a free blog service, identified a "spamalanche" that hit its system, and the company had to dismantle 13,000 spam-filled blogs created in the course of a single weekend.

"The readership of blogs has exploded in the last 18 months," and with it the popularity of splogs, said Jason Goldman, product manager for Blogger, which is owned by Google Inc. "The challenge is one of balance: to make it difficult for people to post bad script but not make it hard for our users."

Unauthorized advertisers are blighting the blogosphere by hijacking legitimate discussions of topics with a flurry of phony comments.

"We would get surges of it -- as many as 200 to 300 within two hours; we couldn't blacklist the [spammers' online] addresses fast enough," Allen said. "It hampers the open conversation that is the very nature of blogs."

Advertisers are also setting up bogus blogs -- what Goldman and others refer to as splogs -- and linking them to numerous other sites to inflate their popularity on search engines. When searchers click on what they think is a relevant site, they end up on imitation blogs full of gibberish and links to ads. Advertisers will pay the spammers every time someone clicks on one of those links.

Ben Popken, keeper of a blog called TheSpunker, recently searched the Internet for Swiss army knives and found himself stymied by splogs. Every time he typed in the topic on a blog search engine, he kept pulling up a site that appeared to be a legitimate blog but was filled with links to other Web sites.

"In one way, it's a tribute to the openness of the blog system. It's kind of ingenious in this diabolical way," Popken said in an interview. "But something like this happening undermines the trust that blogs are based on."

Spammers often use automated software to set up splogs, so since February Google has stepped up its efforts to stop the trashing before it begins, Blogger's Goldman said. Blogger requires a user to enter a code word before setting up a blog and has developed a way of flagging suspected spammers and requiring a similar verification process before they can post comments, he said. Google is further trying to improve its mechanism for identifying junk blogs from legit ones, he said, and only a few bloggers have complained of problems maintaining their blogs.

Yahoo has instituted controls on its free Yahoo 360 blogging software that allow users to limit viewership and comments on their blogs, said company spokeswoman Meagan Busath. "Obviously, Yahoo has had a lot of experience combating spam," because it had to combat a similar problem with exploitation of its free e-mail accounts, Busath said.

Still, some bloggers say the efforts are not keeping up with incoming spam.

John R. Levine, co-author of "The Internet for Dummies," said spam attacks have gotten steadily worse on his blog in the past six months.

"I get more fake comments from gambling sites than all other comments put together," he said. He has had to start requiring e-mail address verification before letting people post comments on his site, weblog.taugh.com. "It makes you look like a doofus. I have this nice blog about e-mail policy, and comments about poker and naked ladies [do] not improve that conversation."

Identifying responsible parties can be difficult, because free blog software programs -- like free e-mail accounts -- do not require identity verification. The culprits tend to be fast-moving, and their handiwork so far is not as debilitating as other forms of online fraud.

"It's rarely worth the resources and time it takes to find them," said Anne P. Mitchell, president and chief executive of the Institute for Spam and Internet Public Policy and a law professor at the Lincoln Law School of San Jose. But Internet companies that helped create the blog phenomenon can also help keep it clean, she said. "From an ethical, moral, good Internet neighbor perspective . . . if they have the ability to do so, they should do so."

These Web sites were created to look like blogs but really just contain links and advertising.