Czech authorities sought a motive on Friday in a student's gun attack at a Prague university, where tearful mourners have left a sea of candles to grieve for 14 victims.

The gunfire on Thursday at the Charles University's Faculty of Arts sparked frantic scenes of students running from the attack that was the Czech Republic's worst shooting in decades.

A makeshift memorial of hundreds of candles lit by tearful students flickered outside the university on Friday as police pursued the investigation at the campus in Prague's historic centre.

"There are 15 dead, which is absolutely unprecedented in the Czech Republic. This is not America," 17-year-old Richard Smaha said "I think it's terrible."

The gunman, a 24-year-old student, killed himself after shooting dead 13 people and wounding 25 others. One of the wounded later died in hospital.

"Those wounded in the gunman's attack at the Faculty of Arts include three foreign nationals, that is, one citizen of the Netherlands and two citizens of the United Arab Emirates," Interior Minister Vit Rakusan told reporters.

Police officer Josef Jerabek said the gunman killed himself when he "felt the loop tightening" as the police approached him after the carnage.

Rakusan had said earlier that there was no link between the shooting and "international terrorism" and that the perpetrator acted on his own.

But police have since detained four people either for threatening to copy the attack or for approving of it.

Police are also still guarding selected sites, including schools, within preventive measures and as "a signal we are here." They will do so at least until January 1, said Rakusan.

The government has declared a national day of mourning on Saturday, with flags on official buildings to be flown at half-mast and people asked to observe a minute's silence at noon.

The gunman, previously unknown to the police, had a "huge arsenal of weapons and ammunition," Police Chief Martin Vondrasek said after the killings on Thursday.

He added an inspection of the crime scene was "the most shattering experience" in his 31 years of police service.

Police started a search for the student even before the mass shooting as they had found his murdered father and the student's note saying he is planning to kill himself in Prague.

They started the search at a Faculty of Arts building where he was expected to attend a lecture, but he went instead to the faculty's main building nearby.

Police learned about the shooting at around 1400 GMT and sent a rapid response unit to the scene. Twenty minutes later, the gunman was dead.

Vondrasek said the gunman's social media account showed he was inspired by a "similar case that happened in Russia," without providing further details.

He added the same gunman had most probably also killed a young man and his two-month-old daughter during a walk in a forest on Prague's outskirts on December 15.

The investigation into those murders, which had shocked Prague, had stalled until evidence found at his home linked the gunman with the crime.

The shooting at Charles University, which sits near major tourist sites like the 14th-century Charles Bridge, was the deadliest since the Czech Republic emerged as an independent state in 1993.

"There is no justification for this horrendous act," Prime Minister Petr Fiala said.

US President Joe Biden sent his condolences, slamming the "senseless" shooting.

"My heart is with those who lost their lives in today's senseless shooting in Prague, those injured, and the Czech people," he wrote on X.

Pope Francis expressed his "spiritual closeness to all affected by this tragedy."

French President Emmanuel Macron, EU chief Ursula von der Leyen and Ukraine President Volodymyr Zelenskyy were among those also offering condolences. (AFP)