PALO ALTO, Calif. (Reuters) - The popular Internet portal Yahoo Inc. (NasdaqNM:YHOO - news) suffered a three-hour outage on Monday after a series of attacks were directed at its computers, blocking millions of people from entering the site to access news and other services.

The crash was the worst in Yahoo's history and stunned many online customers for the way it challenged the increasingly popular notion of the Internet as a reliable system for the exchange of vital information.

Although other popular Internet services like America Online Inc. (NYSE:AOL - news) and eBay Inc. (NasdaqNM:EBAY - news) had similar, or worse outages as they embarked on a course of rapid growth, their problems were related to internal infrastructure rather than sabotage.

The fact that Monday's attack was directed at Yahoo, one of the most visited and most reliable sites on the Internet, raised new questions about the vulnerabilities of all Web sites.

A spokeswoman for Yahoo said that the outage, which began around 1015 PST on Monday, was caused by what appeared to be a planned attack from someone bombarding its servers with fake messages. The effect, she said, was to jam the servers and prevent real users from accessing requested information.

The company stopped short of calling the incident a hacker attack, since it said that no one had actually intruded into its systems. It said none of its internal data or personal customer information had been compromised.

Many questions remained unanswered on Monday, including the source of the attack and why it had been able to cause such a prolonged outage before backup systems kicked in.

Although Yahoo described the outage as ``intermittent,'' users who tried repeatedly to access the site on Monday morning and afternoon said it appeared to be down consistently.

``For the first 15 minutes (of the outage), we were able to get onto the site about half the times we tried,'' said Dan Todd, director of public services at Keynote Systems Inc (NasdaqNM:KEYN - news), a company that monitors Web site performance and reliability.

``But since that time, we got a success rate of less than two percent. To me that's not quite intermittent.''

By 1350 PST, the site was functioning normally.
The type of attack launched against Yahoo, in which large amounts of incoming data jam the site, is a fairly common kind of intrusion, known as a ``coordinated distributed denial of service.''

Monday's attack came through a Sunnyvale, Calif. data center operated by Global Crossing Ltd's (NasdaqNM:GBLX - news) GlobalCenter subsidiary. Most big Internet services use data centers like GlobalCenter to host critical computers and servers in order to make them more secure.

Laurie Priddy, executive vice president of systems applications at GlobalCenter, said the company had worked most of the day on restoring Yahoo's service, and had only started examining the cause of the outage. However, she did say that it appeared the attack had been extensive enough to cover part of the company's backup systems as well as its front line computers.

It was not immediately clear what impact the outage would have on Yahoo's revenues and customer loyalty, although industry analysts noted that Yahoo has become one of the main fixtures of the Internet that many customers do not just surf casually, but depend on in the course of their work days.

``It would be one thing if they were just an Internet search engine,'' says Jupiter Communications analyst Cormac Foster. ''But they provide a lot of mission-critical services like calendars.''

In an effort to distinguish itself from its rivals, Yahoo, and most other Internet portals, have phased in more customized services like online calendaring, where people can go to keep a complete schedule of their appointments. The benefit of such services is that they build viewer loyalty by storing personal data the user can only access by returning to the site. But if service is not reliable, it could have the reverse effect.

Similarly, analysts expect companies that advertise on Yahoo will require some kind of compensation for the outage. Yahoo is the most visited portal on the Internet and every day delivers an average of 465 million pages, many which contain banner ads or some other kind of promotion.

Clay Ryder, vice president and chief analyst at Zona Research, said in addition to the costs of compensating its advertisers, the outage could also cost Yahoo ``soft money'' in the loss of goodwill that could send some advertisers and customers to rival sites.

However, none of Yahoo's big competitors were claiming victory on Monday. Rather, the fact that an established business like Yahoo could be so vulnerable to an outside attack seemed to sweep through the industry as a sobering reminder of the hazards and unknowns still facing all Web sites.