While most appeared to be hoaxes, police were taking every threat seriously to avoid a repeat of Tuesday's shooting, in which a 22-year-old gunman killed 10 people and himself at a vocational school in this town in western Finland.
It was the second school shooting in the country in less than a year. Last November, an 18-year-old man fatally shot eight people and himself at a high school in southern Finland.
"It's clear that the more you talk about these incidents, the more chances there are of copycats," National Police Commissioner Mikko Paatero told reporters in Helsinki. "After we've witnessed two such incidents, the threat is real."
Both gunmen posed with or fired guns in YouTube clips posted before the rampages. The similarities between the attacks - including reports that they bought their guns at the same place - prompted police to probe potential links between them.
"The cases were similar. They were the same type of person, so it could be possible," investigation leader Jari Neulaniemi told The Associated Press. "They had the same style of hair, same kind of clothing, same interests and ideals - and their deeds were the same."
The jitters spread to neighboring Sweden, where one school was evacuated. Police arrested a 16-year-old boy after viewing a suspicious YouTube clip in which he posed with weapons. He was released after police said the video was just a prank.
Police in both countries stepped up their surveillance of YouTube and other Web sites to monitor for signs of possible attack plans.
"We'd be looking to see if we can, by careful analysis, weed out some who would pose a potential threat," said Tero Kuremaa, from the National Bureau of Investigation.
In Finland, police held two young men for questioning about threatening Internet postings, evacuated at least one school and detained a 15-year-old for allegedly sending threats to another school.
Meanwhile, investigators struggled to find the origin of the text messages that sparked fear among students.
"The text messages are threatening in nature and are causing fear and hysteria among young people, and we must stop them," said Urpo Lintula, a spokesman for the regional police department that covers Kauhajoki.
He added that Finland saw a similar wave of threats after the November shooting.
Police, psychiatrists and social workers set up dozens of crisis help lines nationwide to deal with reported threats, calls for help and to provide counseling. The Finnish Red Cross closed down a chat line after 350 callers jammed switchboards.
"There's a lot of uncertainty in the air and there's a lot of fear," said Tero Hintsa, a Red Cross worker in Kauhajoki. Scores of children stayed away from class Wednesday and Thursday, he said.
A 500-student school in the southern town of Keuruu was evacuated after suspicious text messages and Internet postings. A 15-year-old boy was arrested in the west coast city of Turku for allegedly sending threatening messages to a school, STT news agency reported.
Police in the central town of Kajaani detained two men aged 18 and 23 for menacing messages they had posted on the Internet.
"They were fairly vague but they mentioned shootings in schools and bomb explosions," Kajaani police spokesman Arto Lumikari said, adding the men were not believed to be planning any attacks.
On Tuesday, Matti Saari killed eight female students, one male teacher and one male student, and torched their bodies. A 21-year-old woman whom Saari shot in the head was still hospitalized after having two operations.
Sanna Orpana, 17, told AP Television News that she was in the classroom next door when the shooting began at the school.
"We started to hear shooting and a kind of a rumble like tables falling down. We thought someone is playing around, fooling with toy guns," she said, adding that a few students went to look in the other room. "The guy was there with a gun, and tried to shoot them."
Associated Press writers Matti Huuhtanen and Jari Tanner in Helsinki and Karl Ritter in Stockholm, Sweden, contributed to this report.