WASHINGTON — Securities regulators Thursday halted trading in nearly three dozen companies — the initial salvo in Operation Spamalot, a campaign to block e-mails promoting stocks to unsuspecting investors.

The crackdown on investment spam amounts to the biggest such action in the history of the Securities and Exchange Commission. Shareholders lost tens of millions of dollars in the past year by biting on fraudulent Internet offers to "ride the bull" or win "fast money" by buying thinly traded stocks, agency officials said. They continue to investigate whether the spam emanated from third-party stock promoters, corporate insiders or both.

Some of the hyped messages found their way to the e-mail accounts of SEC enforcement lawyers as they spent weeks tracing the alleged scams and their origins. Authorities said the decision to halt trading at 35 penny-stock companies, including a California business that provides computer security services, is merely the first step in a systematic effort to root out the people who sent the misleading stock promotions and others who profited from them.

"When spam clogs our in-boxes, it's annoying," SEC chairman Christopher Cox told reporters. "When it rips off investors, it's illegal and destructive."

Americans each week face at least 100 million spam e-mails hyping stocks, and the number continues to rise steadily, said Oxford University law professor Jonathan Zittrain. The messages work by encouraging people to invest in a stock, thus driving up the price and trading volume in the following days; the culprits sell their shares at a profit and the price plummets.

Pump-and-dump schemes date to Wall Street's early days, but the Internet has transformed the dark, smoky boiler rooms filled with fraudsters making phone calls into an easier, cheaper way to reach potential marks, said SEC enforcement chief Linda Chatman Thomsen.

One of the e-mails investigators released Thursday promotes the "huge news expected out on APPM, get in before the wire." Trading volume on Dec. 18, 2006, the first weekday after the e-mail launch, rose nearly 140-fold, to 484,568 from 3,500 shares, and the stock price rose to 19 cents from 6 cents. Less than two weeks later, the stock of Apparel Manufacturing Associates slid down to 10 cents per share.

Stock trading in each of the companies under regulators' scrutiny, including CTR Investments & Consulting of Pasadena, Md., will be halted for at least 10 days.

CTR sells such things as online visitor sign-in books. Chief executive Jerry Janik, in a phone interview, disavowed any company connection to the e-mails and said the trading shutdown was already causing problems. "This is going to cause me some heartache. It raises eyebrows, which we don't need," he said.