A man detained in connection to a church attack in the southern French city of Nice is believed to have migrated from Tunisia with the suspected Islamist knifeman, sources close to the investigation said today, as Catholics overcame fear to attend church services under heavy security.

Three people were killed in the knife rampage Thursday in the Notre-Dame Basilica that prosecutors say was carried out by a young Tunisian recently arrived in Europe.

It was the latest attack in France to be described by the government as an act of "Islamist" terror, in the wake of the republication of cartoons of the prophet Mohammed by the Charlie Hebdo weekly in September.

In Nice, three men were released from police custody today after authorities determined they were not linked to the suspected attacker Brahim Issaoui, the sources close to the investigation said.

Three men remain in custody, including a 29-year-old Tunisian suspected of travelling with Issaoui from their homeland together. 

The tensions did not prevent Catholics going to church to celebrate the All Saints holiday in Nice, authorities  allowing an exemption during the coronavirus lockdown.

"I was apprehensive, I was scared of coming," said Claudia, 49, as she went to church, reassured by the presence of heavily armed soldiers.

"We need to show that we are not scared and we are here," she said, following several other worshippers into the church, where an early evening mass was held to honour the three victims.

The bishop of Nice, Andre Marceau, said the "abomination of this terrorist act has tainted" the space, lashing at "a perverse, toxic and deadly ideology". 

In a bid to create mutual understanding, a group of Muslim imams and their families attended mass today at the Saint-Esprit de Bagatelle church in the city of Toulouse.

"These people, without mind or reason, want to make another (Koran) interpretation," Lahouary Siali from the Al-Rahma mosque said, referring to violent extremists. "We strongly reject it."

'Came to kill'

Issaoui was shot by police multiple times and is currently in a serious condition in hospital. Investigators have been unable to question him and his precise motivations remain unclear.

But Interior Minister Gerald Darmanin said that Issaoui "had clearly gone there (to Nice) to kill".

"Otherwise how can we explain why he armed himself with several knifes having only just arrived?... He clearly did not come just to get his papers," Darmanin told the Voix du Nord newspaper.

Investigators believe Issaoui travelled to Europe via Italy's Mediterranean island of Lampedusa on 20 September.

The 21-year-old arrived at the mainland Italian port of Bari on 9 October before coming to Nice just two days before the attack.

The latest people to be detained, aged 25 and 63, were arrested Saturday at the residence of the 29-year-old Tunisian, who was detained earlier in the day, a judicial source told AFP.

The detained Tunisian is "suspected of mixing with" Issaoui during their journey to Europe, the source close to the investigation told AFP, adding he also likely arrived in France recently.

Lyon priest shot

France is on edge after the republication in early September of cartoons of the prophet Mohammed by Charlie Hebdo, which was followed by an attack outside its former offices, the beheading of a teacher and the attack in Nice.

On Saturday, an attacker armed with a sawn-off shotgun seriously wounded a Greek Orthodox priest in a shooting outside a church in the French city of Lyon.

Nikolaos Kakavelaki, 52, was closing his Lyon church mid-afternoon when he was attacked and is now in a serious condition in hospital.

The attacker fled the scene and one person was detained. But the man was released today after investigators found no evidence he was linked, prosecutors said.

Prosecutors say they are keeping all hypotheses open but so far have not referred the case to anti-terror colleagues.

French President Emmanuel Macron had vowed after the beheading of teacher Samuel Paty - who showed his class a cartoon of the prophet - that France would never renounce the right to caricature.

This comment prompted a storm of anger in the Muslim world, with furious protests held in numerous countries over the last week.

But Mr Macron, who has vowed to stamp out radical Islamism, sought to quell anger against France by saying in an interview with an Arab TV channel he could understand Muslims could be shocked by the cartoons.

Charlie Hebdo had republished the cartoons to mark the start of the trial of suspected accomplices in the 2015 massacre of its staff by Islamist gunmen. The trial has been suspended until Wednesday after the primary suspect tested positive for coronavirus.