French prosecutors have opened an investigation into a "terrorist plot" after a man known to the authorities as a radical Islamist with mental health troubles stabbed a tourist to death and wounded two other people in central Paris at the weekend before being arrested.

The attack around 9:00 pm (2000 GMT) Saturday near the Eiffel Tower came as France was at its highest alert level against the background of the war between Israel and Hamas.

Terrorism prosecutors told AFP on Sunday they were investigating the knife and hammer attacker, identified as Armand Rajabpour-Miyandoab, a French national born in 1997 to Iranian parents.

Arrested at the scene, he is suspected of murder and attempted murder "in connection with a terrorist plot".

A 23-year-old man, identified by a judicial source as a German-Filipino citizen, died in the attack, though a taxi driver intervened to keep the attacker away from his wife.

Patrick Pelloux, an emergency doctor on duty at the time of the attack, said the couple were both nurses, adding that the woman was severely shocked but unhurt.

A 66-year-old British citizen and a 60-year-old French national were wounded.

Health Minister Aurelien Rousseau told broadcaster France 3 that the wounded victims suffered only "superficial (physical) traumas but of course psychological traumas that will be enormous".

Prosecutors announced a press conference for 7:30 pm on Sunday with updates on the investigation.

Meanwhile, government leaders including Prime Minister Elisabeth Borne and Interior Minister Gerald Darmanin were to hold a security meeting.

"We will not give in to terrorism," Borne wrote on X, formerly Twitter, while President Emmanuel Macron offered his condolences to the family of the man killed.

Afghanistan and Palestine

Rajabpour-Miyandoab, known to authorities for extremism, shouted "Allahu Akbar" — Arabic for "God is greatest" — as he struck, Darmanin said at the scene by Bir Hakeim bridge over the River Seine.

The suspect, who lived with his parents in the Essonne region south of Paris, told police he could not stand Muslims being killed in "Afghanistan and Palestine" and accused France of being "an accomplice to what Israel is doing" in the Gaza Strip, Darmanin added.

German Interior Minister Nancy Faeser condemned the "abominable" crime in an interview with the Funke media group, adding that Berlin's security services were "working closely" with Paris.

"The war in Gaza after Hamas' terrorist act (of 7 October) has worsened the threat," Faeser said, warning that "the threat of Islamist terrorism is acute and serious".

Police and security sources confirmed the attacker had claimed responsibility in a social media video as he struck, speaking about "current events, the government (and) the murder of innocent Muslims".

Investigators would scrutinize his medical history, a security source told AFP, saying the attacker was "very unstable and easily influenced".

Rajabpour-Miyandoab was "being monitored in a way that did not mean he was being hospitalized, he was supposed to follow a course of treatment" for his mental health issues, said Rousseau, the health minister.

"As often in these cases, there's a mixture of an ideology, an easily influenced person and, unfortunately, psychiatry," he added.

Darmanin said the man had previously been sentenced in 2016 to four years in prison for planning another attack in the Paris business district of La Defense, which he failed to carry out.

Help, help

Joseph S., a 37-year-old supermarket manager who asked not to give his last name, witnessed the attack as he sat in a bar. 

He heard screams and people shouting "Help, help" as they ran. A man wielding a hammer attacked a man who had fallen, and within five to 10 minutes the police arrived, he told AFP.

The country has suffered several attacks by Islamist extremists, including the November 2015 suicide and gun attacks in Paris claimed by the Islamic State group in which 130 people were killed.

There had been a relative lull in recent years, even as officials have warned that the threat remains.

But tensions have risen in France, home to large Jewish and Muslim populations, following Hamas's attack on Israel on 7 October and Israel's bombardment of the Gaza Strip.

Security in Paris is also under particular scrutiny as it gears up to host the 2024 Summer Olympic Games.

In October, teacher Dominique Bernard was killed in the northern French town of Arras by a young radicalized Islamist from Russia's Caucasus region.