(Adds quote from student, update from university police, threat at another area university)
By Daniel Lovering
CAMBRIDGE, Mass., Dec 16 (Reuters) - Harvard University on Monday evacuated four buildings at the heart of its centuries-old campus in Cambridge, Massachusetts, after warning students of unconfirmed reports that explosives had been planted.
The Ivy League school said it evacuated the Science Center, Thayer, Sever and Emerson Halls.
"The report remains unconfirmed and the HUPD (Harvard University Police Department) has no reason to believe there is a threat to any other site on campus," Harvard said on its Web site.
Institutions across the Boston area have been on an elevated state of alert since the April 15 bombing attack at the Boston Marathon that killed three people and injured 264.
The Harvard campus police, Cambridge Police and Massachusetts State Police, including the bomb squad, responded to the campus, where they set up yellow-taped perimeters around the evacuated buildings and directed students to move away.
University officials closed Harvard Yard - the wooded and currently snow-covered historic center of the campus - to outside pedestrians, allowing only people who held Harvard identification to enter.
People could be seen walking calmly outside the campus. The school, founded in 1636, has about 21,000 students.
Police did not immediately return calls seeking further details.
Harvard's campus is located in a densely populated urban area adjoining Boston. Students this week were scheduled to take final exams.
Daniel Banks, an 18-year-old freshman from Marlboro, New Jersey, was about to take a final exam in a government class when he was evacuated from Emerson Hall.
"It was my first final ever at Harvard and within a minute it was canceled," Banks said. "I never expected anything like this to ever happen at Harvard especially, but I'm glad that nothing real has happened yet."
It was the latest in a series of security scares at U.S. schools and universities. Three days ago a Colorado high school student intent on confronting a teacher opened fire at his school, severely wounding a classmate before killing himself.
Late last month Yale University in New Haven, Connecticut, placed its campus on lockdown for most of a day after an anonymous caller warned officials that his roommate was headed to the school planning to shoot people. No gunman was found and police now regard the incident as a hoax.
"I've been here eight years and we've had a couple of one-off incidents, but nothing of this scale," said Travis Lovett, 33, of Wakefield, Massachusetts, who works at Harvard. (Additional reporting by Svea-Herbst-Bayliss; Writing by Scott Malone; Editing by Dan Grebler)