PASADENA, Calif. – More than 300 attorneys mustered here yesterday to lay legal siege to Merck & Co., which withdrew its popular anti-arthritis drug, Vioxx, in September because of evidence that the medication caused heart attacks and strokes.

The prospect of a huge new “mass tort” against a leading drug manufacturer clearly energized members of the plaintiffs bar, though they generally maintained a mournful tone in their public remarks to a conference here devoted solely to litigation over Vioxx.

“It’s an unfortunate set of events that have brought us here today,” an Alabama trial lawyer, Andrew Birchfield Jr., said. “The human toll is enormous.”

A recent analysis by the Food and Drug Administration estimated that Vioxx caused 27,000 incidences of stroke or heart attack. Some medical researchers believe the number of serious incidents attributable to Vioxx could be as high as 100,000.

Merck’s decision to pull the drug was a boon to plaintiffs’ lawyers, who had already filed hundreds of cases on behalf of clients who were allegedly hurt by Vioxx. The disclosure this week that the company is also facing a federal criminal investigation left some at yesterday’s Vioxx Litigation Conference, held at the Ritz-Carlton Huntington Hotel & Spa, nearly exultant.

“They’re off the bronco. They’re in the air,” a Louisiana attorney, Vance Andrus, said.

In a statement, Merck denied any wrongdoing and downplayed the coordination by its legal opposition.

“Merck acted responsibly every step of the way with Vioxx,” the statement said. “While trial lawyers position themselves in the Vioxx cases, Merck will continue to manage the litigation in an appropriate, responsible, and measured manner. Merck believes it has strong and meritorious defenses and will continue to vigorously defend itself against allegations brought in any lawsuit.”

One major challenge facing the plaintiffs’ lawyers is that the illnesses thought to be related to the use of Vioxx – heart attack and stroke – are so widespread that it is difficult to prove any particular case was due to the drug.

“Everything that happened after you took a Vioxx pill doesn’t mean that it happened because you took a Vioxx pill,” an attorney from Cherry Hill,N.J., David Jacoby, said.

“That doesn’t mean that Mrs. Smith’s heart attack was caused by the drug. That’s where you get into some real problems.”

As a result, many lawyers are looking for cases involving relatively young patients who became severely ill or died while taking Vioxx but had few pre-existing health problems.

Attorneys who had a case involving a 15-year-old girl who had a heart attack while taking Vioxx became the toast of the conference.

“God bless you guys,” one lawyer said.

Merck has its headquarters in Whitehouse Station, N.J., and lawsuits from Vioxx users all over the country have been filed in the Garden State.

“I think it’s fair to say that New Jersey is the battleground here,” said a Manhattan attorney, Christopher Seeger.

Mr. Seeger said the lawyers for plaintiffs have ample evidence that the company knew about the risks of the drug and was slow to disclose them.

“It’s an overwhelming case where they saw a clear signal, neglected it, and lied about it,” he said.

One of Mr. Seeger’s colleagues, David Buchanan, said the challenge will be proving that Vioxx users, who tend to be older, had problems that were caused directly by the drug.

“A jury is going to be concerned whether it was Vioxx or whether it was 30 years of hard living,” Mr. Buchanan said.

The only speaker offering an industry viewpoint was a Los Angeles lawyer who has represented various drug companies, Thomas Moore. He joked that speaking to the plaintiffs’ lawyers made him feel like “an American soldier detached from your unit in downtown Fallujah.”

“This is first and foremost a publicity effort,” Mr. Moore said. He said the plaintiffs’ lawyers hoped to use news coverage and public attention to force Merck into an early settlement of a large number of cases.

“The idea that the volume of claims can drive the litigation and leverage a big settlement out a drug company like Merck is not going to work,” Mr. Moore said.

That kind of settlement, he said, would mean doling out money to many people who suffered little or no injury from Vioxx.

“My opinion is a global settlement doesn’t inure to anybody’s benefit,” Mr. Moore said.

He also said that tort reform laws and more skeptical juries are making these kinds of cases harder for plaintiffs to win. “Plaintiffs lawyers are no longer seen as the Davids to our Goliath,” he said.

Mr. Moore and several plaintiffs’ lawyers said the outcome of last week’s election was another blow to those who bring product-liability cases. The Food and Drug Administration recently filed briefs backing drug makers in several liability cases. The agency has not yet weighed in on the legal claims over Vioxx.

“There’s a very politically hostile climate to us. We’re not anybody’s favorite,” Mr. Jacoby, the lawyer from New Jersey, said.

A spokesman for the conference organizer, a division of LexisNexis, said it received a flurry of last-minute registrations for the meeting. The fee to attend the one-day session was $795.

One lawyer who is pursuing cases in New York courts, Michael London, said he was impressed with the turnout.

“I’m surprised, but it’s exciting and good,” he said. Mr. London faulted Merck for keeping Vioxx on the market despite repeated warnings that it caused increased incidence of strokes and heart attacks.

“This is a bad drug and they probably should have done this a long time ago,” he said.

Merck has said it cannot provide a reliable estimate for its legal liability related to Vioxx, but some outside analysts have pegged the company’s exposure at up to $18 billion.

Some attorneys said they were concerned that Merck’s stock price has fallen so far since the recall that the company might not be able to pay out all the claims it faces.

“We want their stock price to go up,” one attorney said.