LONDON — As countries across Europe reopen after long Covid-19 lockdowns, movie theaters, pubs and restaurants are looking forward to getting back to business.

But as entertainment and nightlife venues begin to fill up once again, counterterrorism experts and law enforcement officials are warning of a renewed threat: rising domestic terrorism, fueled by an increase in far-right ideology and conspiracy theories that have flourished during the pandemic, or violent Islamist ideology.

“What is likely to happen is just a growing rise of right-wing violent extremism,” Gilles de Kerchove, the European Union’s counterterrorism chief, said in April. “They are using Covid a lot to promote their cause. That is certainly an issue that needs to be at the top of the agenda of governments.”

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The threat of ideologically-driven mass murders looms large in Europe. Last week marked four years since an Islamist bomber killed 22 people outside the singer Ariana Grande’s concert in Manchester, England. This November will mark six years since 90 people were killed at the Bataclan concert hall in Paris.

Since the defeat of Islamic State forces in the Middle East, terrorist attacks across Western Europe have waned. But the threat, police say, may now be growing again.

London’s Metropolitan Police, the United Kingdom’s largest force, said this month that it has thwarted at least four late-stage terrorist plots since the start of the pandemic. The force declined to release details of those cases but said all four were either inspired by extreme right-wing or violent Islamist ideology.

“While the rest of us have been focused on protecting ourselves and our families from Covid-19, terrorists have not stopped planning attacks or radicalizing vulnerable people online,” deputy assistant constable Matt Twist told journalists earlier this month at the Metropolitan Police’s famous Scotland Yard headquarters.

“Now that we’re easing out of lockdown towards normality, we once again need the public’s help in tackling terrorism in all its forms.”

French gendarmes secure the area around the Notre-Dame de l’Assomption Basilica in Nice on Oct. 31, 2020, two days after a knife attacker killed three people.Valery Hache / AFP via Getty Images fileAs a result of the slowdown in public life due to the pandemic, arrests on terrorism charges in the U.K. fell to their lowest level in nearly a decade. According to the Metropolitan Police, 185 people were arrested on terrorism charges since the start of 2020, a 34 percent decline compared to the previous 12 months.

But police are worried that successive lockdowns have created the perfect conditions for terrorism to thrive, fueling long-standing grievances, exacerbating economic inequalities, and stoking distrust of authority.

Moreover, people seeking explanations for these bewildering circumstances may find them in the flourishing conspiracy theory networks on social media, particularly platforms such as Telegram.

“Covid-19 has driven huge numbers of people to spend increased time online,“ Twist said. “And we have seen an increased body of online extremism and hatred, much of which sits below the criminal threshold but which creates a worryingly permissive environment that makes it easier for terrorists to peddle their brand of hatred.”

Put all this together, he said, and you have a situation of “real concern.”

Protestors clash with Metropolitan Police officers during an anti-vaccine rally at Trafalgar Square in London in 2020.Hollie Adams / Getty Images fileWe may already be seeing the consequences. A huge manhunt is underway in Belgium after an armed soldier, who made threats against leading public health experts, went missing last week. The country’s leading virologist and his family are in hiding after he received death threats.

Police fear increased activism by anti-authority, anti-lockdown movements could lead to similar cases, or to incidents like the attacks on 5G cell towers in England in April 2020, which the U.K. government said were “apparently inspired by crackpot conspiracy theories circulating online.”

Potential Islamist plots still make up around 69 percent of the Metropolitan Police’s counterterrorism casework, the force said, but right-wing cases account for 30 percent, and their proportion is growing.

“Radical-right extremism was already increasing prior to the pandemic,” the Centre for Analysis of the Radical Right, a U.K. think tank, wrote in an article published May 25, “but its co-opting of Covid skepticism and conspiracy is highly likely to exacerbate this ever-growing threat.”

Certainly, the pandemic has allowed conspiracy theories and the groups who espouse them to thrive as never before.

“With the emergence of Covid-19, lots of existing conspiracy theory networks in the U.K., which already had links to the far-right as well as the far-left, have really been able to capitalize on this moment,” said David Lawrence, a researcher at the British anti-extremist campaign group Hope Not Hate, which tracks the growth of the far-right across Europe.

“They have reached a new prominence nationally and internationally.”

A man looks at flowers laid outside the Bois d’Aulne secondary school outside of Paris in homage to slain history teacher Samuel Paty, who was beheaded by an attacker for showing pupils cartoons of the Prophet Mohammed in his civics class.Anne-Christine Poujoulat / AFP via Getty Images fileThousands of people, many of whom argue that the increased powers governments have deployed during the Covid-19 crisis are anti-democratic, have attended a series of anti-lockdown marches across Europe over the past year.

Fuelled by semi-public Telegram channels sharing memes, videos, cartoons and strident warnings about those in power, organizers claimed 10,000 attended a march in London in April. (Police would not confirm a figure.)

Many protesters have also espoused anti-vaccine views and expressed strong doubts about the safety of the shots, which have now been given to more than 38 million people in the U.K., more than half the population.

The diverse mix of people — which includes both left-wing and right-wing groups — embodies an unusual blend of anti-authority reaction and long-standing populism, Lawrence said.

“There’s a melding of different groups who are all uniting around this populist, anti-elite, conspiracy theory-inflected agenda,” he said.

“Both conspiracy theories and populist politics share a framework: They divide society between these corrupt, sinister, controlling elites and a pure unknowing people. There’s a binary worldview that gives them a natural fit in some ways.”