Warsaw: A woman wounded three people at an educational institution in Poland's capital, Warsaw, on Saturday, marking the third attack at a school in less than a week, police said.

The attack took place at a private post-secondary school in the Praga district, where the victims were submitting documents at the school office. Bloodstains were visible on the grass near the entrance following the incident.

The suspect used a "dangerous object" to attack a 17-year-old girl and injured two others, aged 17 and 24, Warsaw police spokesman Jakub Filipiak told The Associated Press. None of the injuries was life-threatening, and the suspect was taken into custody.

The latest attack came as police intensified security measures following two other school attacks earlier in the week. Authorities said they had detained 76 people across Poland and inspected nearly 50,000 schools as part of efforts to prevent further violence.

On Wednesday, a 19-year-old student at a vocational school in central Poland attacked fellow students, teachers and passersby with a sharp object, injuring nine people. He was arrested nearby.

The following day, a 15-year-old attacked his former school in the same province, killing a school custodian and injuring two others in a knife attack.

The incidents, both allegedly carried out by Polish nationals, have shocked a country where school violence is considered uncommon.

Police find no evidence of coordinated attacks

The government said it had found no evidence that an online challenge encouraging school attacks was active or that the incidents were connected.

Rumours of coordinated attacks and a so-called "bloody Friday" spread through parents' online groups and mainstream media on Thursday night. As a precaution, 59 schools across Poland suspended classes on Friday, most of them in the Mazovian province.

Other schools introduced additional security measures, including extra staff at entrances and backpack checks.

"There will be no compromise here. The safety of the youth is the most important thing," Interior Minister Marcin Kierwinski said.

Police confiscate knives as concerns grow

Among those detained on Friday were a 14-year-old boy in Warsaw found carrying a concealed-blade knife and 11 firecrackers, and a 16-year-old who brought a butterfly knife to school, police said. Others were detained for posting photographs of knives online or making threats against classmates and schools.

Authorities said enhanced security measures would continue and crisis-response training would be organised. The government also pledged to renew efforts to pass legislation restricting harmful online content.

Prime Minister Donald Tusk said existing laws did not give authorities sufficient powers to block online content showing violent crimes.

In Thursday's attack, the 15-year-old suspect was reportedly wearing a camera to record his actions. School principal Krzysztof Ochmanski eventually overpowered him. The principal later said the boy appeared to realise the gravity of his actions and asked for help.

"Once I had established contact, he asked me for help," Ochmanski said, adding that the situation reached a point where the boy wanted a hug.