BERLIN — Around 9:30 on a quiet Sunday morning late last month, a crudely made explosive device went off with a small bang and a flash in central Berlin near the building of an association of German scientific institutes.

A note found nearby demanded the end to coronavirus restrictions.

Just a few hours earlier, molotov cocktails had been tossed at the front of the Robert Koch Institute, the German federal agency responsible for controlling the virus.

The incidents come against the backdrop of a growing violent undercurrent at large-scale street demonstrations against coronavirus restrictions, including one attended by 20,000 people Saturday in Leipzig. The developments point to an increasingly radicalized movement of virus skeptics in Germany, embraced by the country’s far-right extremist groups and energized by global conspiracy theories, notably those put forth by the U.S.-born QAnon movement.

Far-right groups marched alongside the demonstrators this weekend, stoking concerns among security officials that they will gain recruits and draw more demonstrators to violence, with bomb- and weapon-making material already circulating in coronavirus-skeptic circles online.

“The escalation line is going up all the way,” said Stephan Kramer, president of the state-level intelligence agency in the central state of Thuringia, home to high-profile far-right leaders who traveled to the Leipzig demonstration. “We see with the latest events that there is an escalation toward more violence and to more right-wing extremism among the demonstrations.”

Despite having one of the least stringent lockdowns in Europe during the spring surge in coronavirus cases, Germany has had one of the most vociferous anti-lockdown movements.

The main organizer of the demonstrations, a group called Querdenken, or “lateral thinkers,” grew out of demonstrations in Stuttgart, where large early protests included middle-class moms concerned about vaccines. They oppose mask requirements and what they see as curbs on basic freedoms and have demanded early elections. But the demonstrations have also attracted an array of groups.

In Leipzig on Saturday afternoon, the crowd was mixed: Protesters with heart-shaped balloons and rainbow flags were joined by young men dressed in black, their faces covered with black masks. The black, white and red flag of the pre-1918 German Reich, flown by far-right sympathizers, also fluttered over the crowd.

Police had ordered the gathering in the city’s main square, Augustusplatz, to disperse because participants numbered more than the 5,000 allowed under a court ruling. But as a seething crowd pushed up against police lines chanting, “Peace, freedom, no dictatorship!” officers allowed a stream of demonstrators to march along the city’s ring road as planned.

As night fell, fireworks and other projectiles were hurled at police.

Such incidents at protests send the message to the extreme right that “if you challenge the state, they will give in and we will march,” Kramer said.

Among the assembled were leading figures of the ultranationalist National Democratic Party and the Third Way, said the security office for Saxony state, where Leipzig is located.

“It must be assumed that right-wing extremists will continue to exploit future corona protests for their anti-constitutional goals,” it said. It said the demonstrations are being “intensely evaluated.”

Querdenken’s founder Michael Ballweg has said the movement tried to distance itself from right-wing extremists. There was no violence started by the movement, the group said in statement Saturday, and its social media channels blamed far-left infiltrators.

“It is certainly amazing how a peaceful movement from the center of society is unjustifiably criminalized and stigmatized,” it said.

But experts say that coronavirus skeptics have been particularly susceptible to more radical conspiracy theories, with overlap online between anti-lockdown channels on platforms such as Telegram and groups supporting QAnon, which has ballooned in Germany in recent months.

German supporters of QAnon’s theories believe President Trump was set to save Germany from a global cabal of child abusers. There are around 77,000 unique users in QAnon-associated Telegram chat groups in Germany, according to Miro Dittrich, a researcher at the Amadeu Antonio Foundation focused on tracking online extremist groups.

The theory found fertile ground for support among Germany’s Reichsbürger movement, which rejects the modern German state and has propensity for violence and a tendency for stockpiling weapons, according to German intelligence authorities.

The overlap between coronavirus skeptics and QAnon groups on Telegram is troubling, researchers say. Most posters in online anti-lockdown groups are first-time Telegram users, but some go on to become active in right-wing extremist groups on the platform, according to a study by German television channel NTV and Süeddeutsche Zeitung newspaper that examined 100,000 Telegram accounts.

For coronavirus skeptics, said Josef Holnburger, a researcher whose data was used in study, Telegram is a “gateway to more extremist content.”

Trump’s loss in the U.S. elections will add to frustrations for followers who may be tempted to instead take things into their own hands, Dittrich said.

The loss could “draw people toward more violence,” he said.

There are already concerning signs. On Oct. 31, a member of an anti-lockdown Telegram group that Dittrich monitors posted bombmaking instructions.

Kramer said there have been several cases of such bombmaking and weapons-making instructions being shared on such platforms. “Again, this is part of the whole radicalization process,” he said.

Police said they are still investigating the two incidents in Berlin and are looking at whether there were political motives.

Last week, Bodo Ramelow, the state premier of Thuringia, found a candle on his doorstep of the kind that’s used for graveside memorials along with a flier for an anti-coronavirus demonstration. His home address had been shared in a local anti-lockdown chat group.

“Their message is clear,” he said. “They wished for my end — that I land in a graveyard.”

Germany’s more established far-right extremist groups have been quick to capitalize on anti-lockdown sentiment like they tried to do in 2015 by stirring anti-immigrant sentiment as Germany opened its doors to more than a million refugees.

“It wasn’t so easy to use that situation for their aims,” said Oliver Decker, a professor at the University of Leipzig who studies the far right. “I think it’s now more possible. The pandemic offers another opportunity to mobilize.”

Authorities in Saxony said that most of the 20,000 people who gathered Saturday were peaceful. While 8 in 10 Germans think that strict measures are necessary to contain the coronavirus, according to a poll by ARD television, around 1 in 4 think the restrictions are disproportionate. Germany entered a new month-long lockdown at the beginning of November, as cases surged across Europe.

“We want our children to grow up in freedom,” said Melanie Hoffman, 39, who turned out to demonstrate in Leipzig. “Without masks in schools, without forced vaccinations. To be able to live their lives freely, not in a dictatorship.”

Many are just worried parents, she said.

But the demonstrations give far-right agitators the opportunity to approach a broader part of society that shares central ideas associated with right-wing extremism.

In a news conference Monday, Steve Alter, a spokesman for Germany’s Interior Ministry, said security authorities are still of the opinion that it has not yet been possible for extremists “to be able to completely instrumentalize the protests for their own purposes.”

But they are increasingly visible. And there are concerns that the far-right is finding recruits.

Peter Klug, a 54-year-old who had traveled to Leipzig from Bavaria, shrugged off the Reich flags present, popular among neo-Nazi groups, as “nothing to do with Nazism.”

The presence of far-right groups was not a concern for him, he said. He was only worried about the far-left.

Kramer said that most of the demonstrators know who they are marching alongside. “It seems more or less that they accept each other and consider themselves partners in these demonstrations,” he said. “And that’s very dangerous.”

Morris also reported from Leipzig.