A man has been arrested after three people were injured in a stabbing attack at a train station in Switzerland.

Police said the three victims - Swiss nationals aged 28, 43 and 52 - were hospitalised after the attack at Winterthur train station.

The suspect, a 31-year-old Swiss man, used a bladed weapon, they said, adding that the motive was under investigation.

Switzerland's President Guy Parmelin said on social media he was "shocked by the terrorist attack this morning in Winterthur". "This deeply affects me," he added.

Mario Fehr, Canton Zurich's security director, said it was "clear from the scene that the motive for this act must be sought in the realm of radicalisation and extremism".

Marius Weyermann, commander of the Zurich cantonal police, said at a news conference with Fehr that the suspect lived in the area and had come to the authorities' attention in 2015 for distributing Islamic State (IS) group propaganda.

He said that on Monday, the suspect was sent to a psychiatric hospital after he had presented himself at a police station, where he made "incoherent statements".

The next day, he left the hospital. Officers took him back, and on Wednesday, doctors certified that he no longer posed a danger to himself or others, he added.

Weyermann said one of the victims, the 52-year-old, was stabbed in the thigh and remains in the hospital after receiving emergency surgery.

The 28-year-old and 43-year-old received stab wounds to the leg and neck, respectively, and have either left or are preparing to leave the hospital, he added.

Local media quoted eyewitnesses saying the man held a knife, while people around him were screaming and running away.

An eyewitness working in an office building nearby told a local newspaper he heard a man yell "Allahu Akbar" - meaning "God is greatest" in Arabic - at around 08:30 local time before attacking people with a knife.

A group of schoolchildren were passing through the area at a time, and a school teacher was seen standing in front of them to protect them, according to local media.

Another eyewitness, a taxi driver, told Zurich-based daily Neue Zürcher Zeitung that a man had been walking around the station's underpass, attacking people.