PARIS—Five years ago, police arrested a Frenchman in his 20s who had acquired fake identification and bought paramilitary-style clothing as part of a plan to join militants in Syria.

The suspect, whose first name is Nassim, was convicted of conspiracy to support a terrorist group, under laws that grant French authorities wide latitude to take extremists off the streets, according to his lawyer.

In the coming weeks, he is to be set free, among the first of hundreds of inmates radicalized during the war in Syria and the rise of Islamic State who will be released from French prisons before the end of next year.

With their release, French antiterror authorities are bracing for the resurgence of a security threat that had waned when they broke up Islamic State terror cells in Europe. To try to prevent potential attacks, French police are forming a new unit to monitor former inmates.

“We run a huge risk: seeing people leave prison at the end of their sentences who will not have reformed at all, who are potentially even more extreme as a result of their time inside,” Paris prosecutor François Molins said on national television in May.

That group includes inmates finishing longer sentences who were convicted before the Syrian war, such as Djamel Beghal, who left prison on Monday. Mr. Beghal was handed his first terrorism-related conviction in 2001, another in 2013, and French magistrates said he was a mentor to two of the men who launched the 2015 Charlie Hebdo attacks

Mr. Beghal is being sent directly out of France, to his native Algeria, French media reported.

About 50 people serving terrorism-related sentences and a further 400 classified as “radicalized” while in prison will be released before the end of 2019, according to the Ministry of Justice.

In a country still scarred from a string of deadly terrorist attacks over the past three years—many of them carried out by people who spent time in French prisons—their impending return to society will test the ability of security services to manage the potential threat.

Some inmates with expiring terms were convicted of non-terrorist crimes but embraced radical Islam in prison, the Justice Ministry said. Others were convicted of terror offenses that carried sentences shorter than five years.

In 2016, France passed tougher laws that lengthened sentences for certain terror offenses. In 2017, the average prison sentence in the European Union for terrorism-related crimes was five years, according to Eurojust, an EU agency.

Nassim was given a six-year sentence. Sentences for more serious terrorism offenses carried longer terms. Before his arrest, he was unemployed, espousing extremist views online from his home in the French Riviera city of Nice, according to his lawyer Camille Lucotte.

Nassim still practices a “rigorous” form of Islam, said Ms. Lucotte, who agreed to discuss his case on condition that his last name not be disclosed.

Asked if Nassim remains radicalized, Ms. Lucotte said that “if being radicalized is wanting to commit terrorist acts, no.”

French authorities are worried that freed extremists may still support violence.

“We will anticipate their release and follow them extremely closely,” Minister of Justice Nicole Belloubet told national TV last month.

People fresh out of French prisons have committed notorious terrorist attacks in recent years, including a 19-year-old who killed a priest at a church in northern France and a man who stabbed a police captain and his companion at their home outside Paris.

The number of extremists in French prisons has grown in recent years alongside the rise of Islamic State, the Justice Ministry said. As it gained territory in Iraq and Syria, recruiters and online propaganda drew thousands of followers to the group’s cause.

A French crackdown caused the number of extremists in prison on terrorism-related offenses to surge; these inmates mingled with the broader prison population, raising the possibility that more people could be radicalized.

There are currently 512 people imprisoned in France for terrorism-related crimes, according to the Justice Ministry, a more than threefold increase in four years. Some 1200 other prisoners are deemed to be “radicalized,” a 70% increase since 2015, according to government data.

Intelligence services have struggled to monitor the large number of people quickly radicalized online by Islamic State and its sympathizers, according to Jean-Louis Bruguière, a former counterterrorism magistrate.

“We had a surge in [potential terrorists] that was unprecedented in our country,” he said. “It was exponential.”

Those up for release include people who were convicted before the rise of Islamic State—and some have radicalized other inmates, French authorities said.

Mr. Beghal, the Algerian man released on Monday, had received a 10-year sentence in 2013 for planning to help Ali Belkacem, convicted of bombing a Paris metro station in 1995, escape from prison; the plan was never carried out.

Mr. Beghal had earlier been sentenced to 10 years for a 2001 plot to bomb the American Embassy in Paris.

The new unit within UCLAT, the antiterrorism division of France’s police force, will try to prevent such radicalizations. The unit will share information between prison services and intelligence agencies, and review details such as watchlisted peoples’ plans to change address or travel abroad, according to an adviser to the interior minister.

“The stakes are much higher for government because people are focusing on this issue” after living through the attacks of recent years, said Farhad Khosrokhavar, a sociologist at the École des Hautes Études en Sciences Sociales and the author of a new book on jihad. “Now the government is much more afraid than before.”

By Josh Jacobs and Matthew Dalton

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