A lone gunman opened fire Thursday in a university building in downtown Prague, killing at least 15 people and injuring more than 20 in the Czech Republic’s worst mass shooting, police and the city’s rescue service say.

The bloodshed took place in the philosophy department building of Charles University, where the shooter was a student, Prague Police Chief Martin Vondrasek said. The gunman also died. His name has not been released.

Mr. Vondrasek said 24 were injured, and authorities warned that the death toll could rise.

Police gave no details about the victims or a possible motive for the shooting at the building located near the Vltava River at Jan Palach Square. Czech Interior Minister Vit Rakusan said investigators do not suspect a link to any extremist ideology or groups.

Mr. Vondrasek said police believe the gunman killed his father in his hometown of Hostoun, just west of Prague, earlier in the day, and that he had also been planning to kill himself. He didn’t elaborate.

The chief described the shooter as an excellent student with no criminal record, but didn’t provide any other information.

The gunman suffered “devastating injuries” but it wasn’t clear if he killed himself or was shot to death in an exchange of gunfire with officers, Mr. Vondrasek said, adding that there was “nothing to suggest that he had an accomplice.”

The shooter legally owned several guns, and what he did was “well thought out, a horrible act,” Mr. Vondrasek said.

“We mourn the loss of life of members of our university community, express our deepest condolences to all the bereaved and our thoughts are with all those affected by the tragedy,” Charles University said in a statement.

University authorities said they would tighten security in university buildings with immediate effect.

The building where the shooting took place is near the Vltava River at Jan Palach Square, a busy tourist area in Prague’s Old Town. It is just a few minutes walk from the picturesque Old Town Square, a major tourist attraction where a popular Christmas market attracts thousands of visitors.

The government quickly sought to quell concerns that the massacre was backed by foreign interests.

“There’s no indication that it has anything to do with international terrorism,” Mr. Rakusan said.

“It’s a horrible crime, something the Czech Republic has never experienced,” he said.

Pavel Nedoma, the director of the nearby Rudolfinum Gallery, said he watched from a window as a person standing on a balcony of the building fired a gun.

Authorities evacuated everyone from the building and police said they were still searching the area, including the balcony, for explosives.

The building forms part of the square and faces a bridge across the river with a view of Prague Castle, the seat of the Czech presidency. President Petr Pavel said he was “shocked” by what happened and offered his condolences to the relatives of the victims, as did leaders of Germany, France and Slovakia, the European Union and Israel.

Where tourists, students and others would normally be enjoying the view of the iconic monument, chaos and terror instead took hold. Police vehicles and ambulances sped across the bridge with their sirens wailing. Officers sealed off the empty square.

Some video footage showed people being evacuated from the building and others trying to hide by a wall.

The Czech government planned to meet later Thursday for an emergency session to discuss the shooting.

Before Thursday, the nation’s worst mass shooting was in 2015, when a gunman opened fire in the southeastern town of Uhersky Brod, killing eight before fatally shooting himself.

Associated Press