(Adds San Francisco incident, paragraph 5-6)

WASHINGTON, April 18 (Reuters) - A series of bomb threats and other security alerts rattled U.S. schools and universities on Wednesday as nerves remained frayed from the massacre of 32 people by a disturbed, brooding student at Virginia Tech.

With the Virginia Tech campus still on edge, students there got another scare on Wednesday when police swarmed into a building housing the university president's office. An initial report of suspicious activity turned out to be a false alarm.

The campus remains in shock after Cho Seung-Hui, a 23-year-old student at the school, went from classroom to classroom shooting people and then took his own life on Monday in the bloodiest shooting spree in modern U.S. history.

Students have been the main victims in several recent U.S. shootings, most infamously the 1999 Columbine High School massacre in Colorado in which two student gunmen killed 12 other students and a teacher before killing themselves.

In central San Francisco, the University of California Hastings College of the Law evacuated 300 to 400 students midway through classes on Wednesday afternoon after someone noticed a posting on an Internet site referring to the school and Virginia Tech, Dean Nell Jessup Newton said.

"Somebody who read that bulletin board called us. You could read it as sort of a sick joke or you could read it as something that was serious," she told Reuters. "We called the police and the FBI and they are investigating, but in light of the Virginia Tech situation we decided in a kind of excess of caution to close down the school for the rest of the day."

At the University of Minnesota in Minneapolis, eight buildings were evacuated on Wednesday after a bomb threat, the school said.

Classes in the buildings were canceled for the rest of the day and students were asked to return to residence halls and not congregate in the area, the university said in an announcement on its Web site.

A high school and a middle school in Columbia, Missouri, were locked down on Wednesday afternoon after gunfire was exchanged between two cars nearby, and two people were taken into custody, the Columbia Tribune reported.

The University of Missouri said in a notice on its Web site the shooting did not take place on its campus.

At Vista Murrieta High School in Murrieta, 50 miles (80 km) south of Los Angeles, graffiti threatening that everyone would die on April 20 -- the anniversary of the Columbine shootings -- and that bombs had been planted was found on Wednesday. The campus was briefly evacuated and security was beefed up, local media reported.

San Diego State University received an apparently bogus threat overnight to carry out a Virginia Tech-style killing spree. Staff alerted police and a suspect was being questioned but campus activities continued as normal.

In the Denver area, three schools received threats against specific students or of bomb threats. One was locked down briefly, one briefly evacuated and another was put on security alert. Nothing was found in any case, The Denver Post reported.

An 18-year-old student at the Lawrenceville School, a boarding school for high school students in Lawrenceville, New Jersey, was taken into custody by police on Wednesday afternoon after he waved an air gun out of a dormitory window. (Reporting by Adam Tanner in San Francisco, Jon Hurdle in Philadelphia, Jill Serjeant in Los Angeles, Michael Conlon in Chicago)

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