A Greek Orthodox priest was shot on Saturday while he was closing his church in the French city of Lyon, police said.
He has been named locally as Fr Nikolaos Kakovelakis.
The priest, a Greek citizen, is in a local hospital in a critical condition with life-threatening injuries after being shot twice in the abdomen, a police official told The Associated Press.
The attacker was alone and fired from a hunting rifle, said the official.
Police cordoned off the largely residential neighbourhood around the church, and detained one person who resembles descriptions of the gunman, but was not armed at the time of his arrest, the Lyon prosecutor said in a statement.
As night fell on Lyon, an Associated Press reporter saw police tape and emergency vehicles throughout the neighbourhood.
National police tweeted that "a serious public security incident" was under way.
The reason for the shooting was unclear.
It happened two days after a knife attack at a Catholic church in the French city of Nice that killed three people, and amid ongoing tensions over a French newspaper's publication of caricatures mocking the Muslim Prophet Muhammad.
French anti-terrorist authorities were following the case but not investigating Saturday's shooting.
The interior minister activated a special emergency team to monitor the manhunt, and the Lyon prosecutor opened an investigation for attempted murder.
"No theory is favoured, no theory is ruled out," Lyon mayor Gregory Doucet told reporters at the scene.
"We don't know at this stage the motive for this attack."
Antoine Callot, the pastor at another Greek Orthodox church in Lyon, identified the wounded priest as Nikolas Kakavelakis, a 45-year-old father of two, the Associated Press reported.
Mr Callot told the AP that the Greek Orthodox community in Lyon has not received any threats, but said he immediately asked police for security protection at his church after the shooting.
"We are anxious and anguished. It's really horrible," he said. "Now we need to hide and be careful."
Residents and a local police patrol heard shots near the church, and when officers arrived they saw an individual running away and found the wounded priest by the back door of the church, the Lyon prosecutor said in a statement.
Prime Minister Jean Castex reiterated government promises to deploy military forces at religious sites and schools.
He said French people can "count on the nation to allow them to practise their religion in full safety and freedom".
Seeking to calm tensions and to explain France's defence of the prophet cartoons, President Emmanuel Macron gave an interview broadcast on Saturday on Arabic network Al-Jazeera.
Mr Macron also tweeted that "our country has no problem with any religion", adding: "No stigmatisation: France is committed to peace and living together in harmony".
The Greek Foreign Ministry said in a statement: "We condemn the attack against an Orthodox priest of Greek origin near the Church of the Annunciation in Lyon, France. The Ministry of Foreign Affairs is in constant contact with the competent French authorities."