An 84-year-old priest was forced to the ground before two attackers slit his throat at his church in Normandy, a nun who was in the church said.
The attack near the city of Rouen, in which Father Jacques Hamel died, was claimed by the Islamic State group.
The nun, identified as Sister Danielle, said: "They forced him to his knees. He wanted to defend himself. And that's when the tragedy happened."
"They recorded themselves. They did a sort of sermon around the altar, in Arabic. It's a horror," she told BFM television.
Of her fallen colleague, she said: "He was a great priest."
One person has been detained in the investigation into the attack in the small north-western town of Saint-Etienne-du-Rouvray.
A spokeswoman for the Paris prosecutor's office said the person was detained following the incident in which Father Hamel was killed during Mass.
The Paris prosecutor's office oversees investigations involving terrorism.
The attackers took hostages during morning Mass, slitting the priest's throat before being killed by police.
Authorities are trying to determine whether they had accomplices.
A resident of St-Etienne-du-Rouvray says he heard shots fired for about 30 seconds after police responded to an attack inside the church.
Claude-Albert Seguin, a 68-year-old pensioner, said he knew the priest who was killed, adding: "Everyone knew him very well. He was very loved in the community and a kind man."
He added that police told all neighbours to shutter their windows.
The Islamic State group issued a statement published by the IS-affiliated Aamaq news agency which said the attack was carried out by "two soldiers of the Islamic State."
It added the attack was in response to its calls to target countries of the US-led coalition which is fighting IS.
Archbishop Dominique Lebrun of Rouen confirmed the identity of the priest as Father Hamel.
A local Muslim leader said one of the men who attacked the church was on French police radar and had travelled to Turkey.
Mohammed Karabila, president of the Regional Council of the Muslim Faith for Haute-Normandie and head of the local Muslim cultural centre, said: "The person that did this odious act is known, and he has been followed by the police for at least a year and a half."
He said the attacker "went to Turkey and security services were alerted after this."
Mr Karabila said he hoped that interfaith dialogue in his region would not be damaged.
French president Francois Hollande called the incident a "vile terrorist attack" and said it is another sign that France is at war with IS, which has claimed a string of attacks on France.
"We must lead this war with all our means," he said in Saint-Etienne-du-Rouvray.
Mr Hollande expressed support for all France's Catholics but said the attack targets "all the French."
Another person inside the church was seriously injured and is hovering between life and death, said Interior Ministry spokesman Pierre-Henry Brandet.
Police rescued three people from the church, said Mr Brandet. The hostage-taking occurred during morning Mass, he told reporters.
Pope Francis condemned the attack on the church in the strongest terms.
Vatican spokesman, the Rev Federico Lombardi, said the attack hits particularly hard "because this horrific violence took place in a church, a sacred place in which the love of God is announced, and the barbaric murder of a priest and the involvement of the faithful."
Rev Lombardi called the attack "more terrible news, that adds to a series of violence in these days that have left us upset, creating immense pain and worry."
He said the pope has expressed "pain and horror for this absurd violence, with the strongest condemnation for every form of hatred and prayer for those affected."
The Vatican expressed its closeness to the Roman Catholic Church in France and the Archdiocese of Rouen, as well as to the affected communities and the people of France.
In a statement from Krakow, Poland, where Pope Francis was visiting, Archbishop Lebrun said: "I cry out to God, with all men of good will. And I invite all non-believers to unite with this cry ... The Catholic Church has no other arms besides prayer and fraternity between men."
The identities of the attackers and motive for the attack are unclear, according to a security official.
The slaying "is obviously a drama for the Catholic community, for the Christian community," said Mr Brandet.
He added that the RAID special intervention force was searching the church and its perimeter for possible explosives and terrorism investigators had been summoned.
France is on high alert after an attack in Nice on Bastille Day - July 14 - that killed 84 people and a string of deadly attacks last year claimed by the Islamic State group that killed 147 others.
The country is also under a state of emergency and has extra police presence in the wake of the Nice attack in which a man barrelled his truck down the city's famed Promenade des Anglais, mowing down holiday crowds.
Islamic State extremists have urged followers to attack French churches and the group is believed to have planned at least one church attack earlier.
An Italian politician is urging Pope Francis to put the slain French priest on a fast track for sainthood.
Roberto Maroni, the president of the Lombard region, said in an appeal circulated on social media that "Father Jacques is a martyr of faith" and requested that the pope "immediately proclaim him St Jacques."
Shortly after the appeal, the hashtage #santosubito, which translates as "saint immediately," began circulating on Twitter.
The canonisation process is a lengthy one involving two miracles attributed to the person's intercession, but in the case of a martyr only one miracle is needed, after beatification.
There must first be a declaration by the Vatican that the person indeed died for the faith.
The United States has condemned the attack in the "strongest possible terms".
White House spokesman Ned Price says France and the United States are committed to protecting religious liberty for all faiths and the commitment will not waver because of the attack.
Mr Price also commended what he said was the "quick and decisive response" by French law enforcement officers.