No one could help but notice the police presence at Sunday night's Yehudit Ravitz concert at San Francisco's Masonic Auditorium.

But an excited 1,400 fans of the Israeli rock singer celebrated Jerusalem 3000, undeterred by threats that shook the American Jewish community two days earlier.

The so-called "International Freedom Fighters Association," which threatened to kill 1,200 Jewish American executives and doctors last week, did not target the event — much to the relief of the sponsoring organizations, which included the S.F.-based Jewish Community Federation, Jewish Community Relations Council, Israel Project and Israel Consul General.

The anti-Jewish threat was followed by a warning of reprisals against U.S. Muslims, supposedly by an England-based chapter of the Jewish Defense League.

Meanwhile, Bay Area Jewish organizations have beefed up security. At the Ravitz concert, police and security personnel patrolled the auditorium's halls and entrance.

"There is a heightened sense of security. But, at the same time, we're urging members of the community not to feel alarm. Events and activities will continue uninterrupted," said Rabbi Doug Kahn, executive director of the S.F.-based JCRC.

On Friday of last week, a group calling itself the International Freedom Fighters Association sent a letter — postmarked in San Francisco — to the San Jose Mercury News. It said 1,200 Jewish executives and doctors would be killed unless Israel withdrew its military forces from Lebanon and paid $12 billion in compensation with French coordination by May 5.

The threat was an apparent reference to the Israeli soldiers stationed in the security zone maintained by Israel in southern Lebanon and to last month's 16-day assault against Hezbollah targets in Lebanon.

The Mercury News turned over the letter to the FBI, which issued a warning to Jewish organizations and synagogues that day.

The deadline passed without incident. Nonetheless, leaders of the Bay Area Jewish community are taking precautions — including increasing security at such events as Sunday's concert.

"The response of the police departments throughout the area has been exemplary and we continue to count on their effective response to our community's concerns," said Kahn.

Additional security measures include a police car stationed outside the Jewish Community Center of San Francisco at 3200 California St.

"We hope this will bring comfort to the parents of nursery school students here," said Wayne Feinstein, executive vice president of the S.F.-based Jewish Community Federation, the umbrella organization of local Jewish agencies. "We take this threat seriously. But we won't let it disrupt our work."

By boosting security measures, Kahn added, "The central message is that we are working very closely with the police."

Kahn said he and Rabbi Jacob Traub of S.F. Congregation Adath Israel coincidentally had met new S.F. Police Chief Fred Lau the day before the threat was issued.

Kahn declined to further detail increased security measures, as did Barbara Bergen, director of the Anti-Defamation League's Central Pacific region, and the Consulate General of Israel in San Francisco.

Other agencies said their established security measures would suffice for now. Mount Zion Hospital of S.F. has not increased its security staff, according to Pete Balesteri, director of security.

"There is a heightened awareness. However, at this point we're confident in our camera system and security staffing," he said.

Meanwhile, the FBI continued to investigate the threat. The FBI's San Francisco office said earlier this week it had found no new information about the International Freedom Fighters Association.

After the threat last week, an FBI statement said: "There is no way to know at this time if the threat is real, but the FBI believes that Jewish organizations, synagogues and individuals should for their own safety be on heightened alert."

Bergen and others have suggested the organization — which the FBI had never heard of before last Friday — may be an individual.

"It wouldn't be a surprise. The Unabomber used to write in the royal `we.' It's quite possible for one person to make himself sound like a group," she said.

A source who saw the letter said that the author or authors wrote: "What do you expect from the people who crucified Christ?"

In the letter, the source noted, the author or authors also capitalized "WAR," which often refers to the anti-Semitic, white supremacist group White Aryan Resistance.

For now, Jewish community leaders agreed to proceed with caution.

"Unfortunately Jewish organizations have been targeted in the past, so we can't dismiss these things out of hand," Bergen said. "I think the Jewish community is as security-conscious as any group can be. Perhaps even more so.

"But obviously the May 5 deadline passed and nothing happened. What conclusions can be drawn are anyone's guess."

Malcolm Hoenlein, executive vice chairman of the Conference of Presidents of Major American Jewish Organizations, said there is no undue "reason for alarm" at this time. However, Jewish groups must repudiate attempts to "hold us hostage," he said.

The Presidents' Conference and its member groups were already on alert after the militant Islamic groups Hamas and Hezbollah issued threats to strike at Jewish targets worldwide, Hoenlein said.

Earlier this week the FBI in New York and Washington, D.C., also began investigating a series of anti-Muslim letters to Muslim organizations in California, New Jersey, New York, Virginia and Washington, Reuters reported.

The FBI's Joint Terrorist Task Force was investigating the letter, allegedly sent by a JDL-Kahane Chai group from Birmingham, England. A JDL spokesman in New York denied the group was behind it.

According to Reuters, the letter said: "Our arm is long and if our people are attacked outside Israel, we will respond against the Muslim community around the world…our units are already in place and trained.

"Be warned Muslims," the letter continued. "Your lives are ours like sheep to the butcher…you will be delivered into our hands."