Twenty-five people have been arrested in raids across Germany on suspicion of plotting to overthrow the government.
The group of far-right and ex-military figures are said to have prepared for a "Day X" to storm the Reichstag parliament building and seize power.
A man named as Heinrich XIII, from an old aristocratic family, is alleged to have been central to their plans.
According to federal prosecutors, he is one of two alleged ringleaders among those arrested across 11 German states.
The plotters are said to include members of the extremist Reichsbürger [Citizens of the Reich] movement, which has long been in the sights of German police over violent attacks and racist and antisemitic conspiracy theories.
They also refuse to recognise the modern German state.
Other suspects came from the QAnon movement who believe their country is in the hands of a mythical "deep state" involving secret powers pulling the political strings.
Interior Minister Nancy Faeser assured Germans that authorities would respond with the full force of the law "against the enemies of democracy".
An estimated 50 men and women are said to have been part of the group, which allegedly plotted to overthrow the republic and replace it with a new state modelled on the Germany of 1871 - an empire called the Second Reich.
"We don't yet have a name for this group," said a spokesperson for the federal prosecutor's office. The interior minister said it was apparently made up of an organisation "council" and a military arm.
Wednesday's dawn raids are being described as one of the biggest anti-extremism operations in modern German history.
Three thousand officers took part in 150 operations in 11 of Germany's 16 states, with two people arrested in Austria and Italy.
Almost half of arrests took place in southern states of Baden-Württemberg and Bavaria. More than one in five Reichsbürger are thought to be based in the south-western state of Baden-Württemberg alone.
Justice Minister Marco Buschmann tweeted that a suspected "armed attack on constitutional bodies was planned". Faeser said later that the investigation would peer into the "abyss of a terrorist threat from the Reichsbürger scene".
The suspects
The federal prosecutor's office said the group had been plotting a violent coup since November 2021 and members of its central "Rat" (council) had since held regular meetings.
They had already established plans to rule Germany with departments covering health, justice and foreign affairs, the prosecutor said. Members understood they could only realise their goals by "military means and violence against state representatives", which included carrying out killings.
Investigators are thought to have got wind of the group when they uncovered a kidnap plot last April involving a gang who called themselves United Patriots.
They too were part of the Reichsbürger scene and had allegedly planned to abduct Health Minister Karl Lauterbach while also creating "civil war conditions" to bring about an end to Germany's democracy.
A former far-right AfD member of the lower house of parliament, the Bundestag, is suspected of being part of the plot, and of being lined up as the group's justice minister.
Birgit Malsack-Winkemann, who was among the 25 people arrested, returned to her role as judge last year and a court has since turned down attempts to dislodge her.
A prominent lawyer was pencilled in to handle the group's foreign affairs, with 71-year-old Heinrich XIII as leader.
Public Prosecutor General Peter Frank said Heinrich was among the suspects whom investigating judges had asked to be held in custody.
Heinrich XIII styles himself as a prince and comes from an old noble family known as the House of Reuss, which ruled over parts of the modern eastern state of Thuringia until 1918.
Descendants still own a few castles and Heinrich himself is said to have a hunting lodge at Bad Lobenstein in Thuringia.
The rest of the family have long distanced themselves from the minor aristocrat, with one spokesman telling local broadcaster MDR during the summer that Heinrich was an "at times confused" man who had fallen for "misconceptions fuelled by conspiracy theories".
As well as a shadow government, the plotters allegedly had plans for a military arm run by a second ringleader identified as Rüdiger von P.
They were made up of active and former members of the military, officials believe, and included ex-elite soldiers from special units. The aim of the military arm was to eliminate democratic bodies at local level, prosecutors said.
Alleged recruitment plans
Rüdiger von P is suspected of trying to recruit police officers in northern Germany and of having an eye on army barracks too. Bases in the states of Hesse, Baden-Württemberg and Bavaria were all inspected for possible use after the government was overthrown, officials said.
One of those under investigation had been a member of the Special Commando Forces, and police searched his home and his room at the Graf-Zeppelin military base in Calw, south-west of Stuttgart.
Another suspect has been identified as Vitalia B, a Russian woman who was asked to approach Moscow on Heinrich's behalf. The Russian embassy in Berlin said in a statement that it did not "maintain contacts with representatives of terrorist groups and other illegal entities".
Several violent attacks have been linked to Germany's far-right in recent years. In 2020, a 43-year-old man shot dead nine people of foreign origin in the western town of Hanau, and a Reichsbürger member was jailed for killing a policeman in 2016.
The Reichsbürger movement is estimated to have as many as 21,000 followers, of whom around 5 percent are considered to belong to the extreme right.
Analysis by BBC Berlin correspondent Jenny Hill: Reichsbürger - German 'crackpot' movement turns radical and dangerous
The Reichsbürger were, for years, a source of national derision, dismissed as crackpots.
But they're increasingly a source of concern for the security services who say they are becoming more radical and more dangerous.
Members do not recognise the post-war German state and reject the authority of its government. Despite the name, which translates to Citizens of the Reich, this is no organised national movement - rather a disparate set of small groups and individuals scattered across the country who are united in that shared belief.
Some print their own currency and identity cards and dream of creating their own autonomous state.
Earlier this year for example, a group calling itself the Königreich Deutschland (Kingdom Germany) bought two pieces of land in Saxony upon which they intended to create their own self-administered state.
Others refuse to pay tax or intentionally clog up the administration of local authorities by sending large volumes of, often abusive, letters.
And many have guns - legally or otherwise.
Since 2016, when a Reichsbürger shot and killed a policeman as officers raided his stash of weapons, the German authorities have revoked more than a thousand gun licences of people they believe to subscribe to the ideology. But at the end of last year around 500 still had valid gun licences.
Government figures show that Reichsbürger and so-called Selbstverwalter - a "grouping" with similar beliefs that translates as self-administrators - committed more than 1000 extremist criminal acts in 2021, double the number in 2020.
Of the 21,000 people in the Reichsbürger "scene", around 5 percent are believed to be right-wing extremists and 10 percent potentially violent.
They also have links to the German military, says Miro Dittrich, an expert who tracks the group as well as other conspiracy theorists.
The pandemic, he believes, has served to further radicalise the group as well as increase support.
"The pandemic was a hard moment for a lot of people. It was unclear how things were going to develop… conspiracy narratives were quite attractive for a lot of people because it gave the world an order," he said.
Reichsbürger members demonstrated alongside anti-vaxxers and Covid-19 deniers (indeed some share those positions) as well as QAnon supporters during mass street protests in the last few years. They were there when a mob from a Covid-19 demonstration tried to storm the Bundestag in August 2020.
Many have been surprised at the depth to which conspiracy theories in general have permeated German society - particularly during the pandemic. I've experienced it myself while reporting on various demonstrations over the last few years.
Plenty of people have told me that the Covid-19 vaccine is a poison or repeated the commonly voiced conviction that the German government plans to "replace" the "native" population with foreigners.
Just last week, at a small protest against high energy bills, one man expressed the view that Germany's foreign and economy ministers were preparing to declare war on their own country.
What worries the authorities is the extent to which that could boil over into violence. Of course not every conspiracy theorist represents a physical danger. But plenty of politicians here have been on the receiving end of death threats. Plenty more are agonising over how to stop the spread of disinformation.
So today's raids and arrests will be the source of huge national concern.
-BBC