The city of Borovichi, with a population of some 70,000, was put on alert at 11:30 p.m. local time (20:30 GMT) with the wail of air raid signals and announcements of an imminent airstrike, the spokesman said.
"The long signal of the siren and the subsequent announcements not only alerted residents in the center of the city, but in the outskirts as well," he said.
Many residents took the signal seriously and police received a large number of calls asking how serious the threat was and what people should do in the event of an air raid.
City police eventually turned off the siren, which is located on top of a dormitory. They are investigating whether a prankster set off the alarm or a malfunction caused the signal to turn on by itself.
The emergency siren is operated by the Emergency Situations Ministry.