One person died and two police officers were seriously injured in a knife attack in eastern France on Saturday that occurred during a demonstration, the local prosecutor said.

Three more officers were lightly wounded in the attack in the city of Mulhouse, carried out by a 37-year-old suspect who is on a terror prevention watchlist, prosecutor Nicolas Heitz told AFP.

The list, called FSPRT, compiles data from various authorities on people with the aim of preventing "terrorist" radicalization. It was launched in 2015 following deadly attacks on satirical magazine Charlie Hebdo's offices and on a Jewish supermarket.

The suspect attacked local police officers in Mulhouse shouting "Allahu Akbar" ("God is greatest") Saturday afternoon, France’s national antiterror prosecutors’ unit PNAT said in a statement.

A passerby was killed trying to intervene and help police, the prosecutor's office said.

One of the seriously wounded police officers sustained an injury to the carotid artery, and the other to the thorax, Heitz said.

Interior Minister Bruno Retailleau was expected to travel to the scene of the attack later Saturday.

Police established a security parameter after the attack, which happened shortly before 4 p.m. local time during a demonstration in support of Congo.

According to union sources, the suspect, born in Algeria, has been under judicial supervision and house arrest, and under an expulsion order from France.

"Horror has seized our city," Mulhouse Mayor Michele Lutz said on Facebook. The incident was being investigated as a terror attack, she said, but "this must obviously still be confirmed by the judiciary."

French President Emmanuel Macron said Saturday that the deadly knife attack was "Islamist terrorism," after France's anti-terrorism prosecutor's office confirmed it was investigating the case.

"It is without any doubt an act of Islamist terrorism," Macron told reporters on the sidelines of the annual French farm show, adding that the interior minister was on his way to Mulhouse.

The suspect has been arrested, the prosecutor's office said.

Some information in this report is from Reuters.