Germany's domestic intelligence agency will monitor leading protesters against Covid restrictions, the interior ministry said Wednesday, over concerns they pose a threat to democracy and have ties to right-wing extremism.

The monitoring will focus on members of Germany's "Querdenker" (Lateral Thinkers) movement, which has emerged as the loudest voice against coronavirus curbs and an active promoter of conspiracy theories that deny basic facts about the pandemic.

Some protest organisers "have clearly demonstrated that their agenda goes beyond simply mobilising protests against the government's corona measures", a ministry statement said. 

Their main aim appears to be to "permanently undermine trust in state institutions and their representatives", it added.

They are also suspected of seeking links with right-wing extremists such as "Reichsbuerger" (Citizens of the Reich) who question the legitimacy of the modern Federal Republic of Germany, and of spreading anti-Semitic messages and QAnon myths, the ministry said.

The extremists encourage supporters to ignore official orders and challenge the state monopoly on the use of force, it added.

After the agency's announcement, local media reported that the BKA federal police force informed MPs who last week voted for a tightening of coronavirus restrictions that their names were circulating on the Telegram messaging app under the title "death list of German politicians".

"It is the message of a single person. That does not result in an increased threat level," the BKA wrote in the letter seen by news agency DPA.

"Querdenker" demonstrations over the past year have attracted thousands, at times tens of thousands, of supporters, where anti-vaxxers and conspiracy theorists are seen marching side by side with neo-Nazis and members of the far-right AfD party.

The rallies are notorious for being broken up early by police because protesters fail to comply with rules on social distancing and mask wearing, and have occasionally descended into clashes.

Interior Minister Horst Seehofer told reporters that the monitoring was aimed at "preventing problems" and "preventing crimes".

For intelligence officers to be legally allowed to start observing parts of the anti-corona movement, Germany's Federal Office for the Protection of the Constitution (BfV) had to create an entirely new category of group because the "Querdenkers" do not fit neatly into the existing classifications of right-wing, left-wing or Islamic extremism.

The new category is for groups suspected of being "anti-democratic and/or delegitimising the state in a way that endangers security".

The designation allows intelligence officers to gather data about individuals and their activities, and can in a further step include shadowing people and tapping their communications.

The BfV is already monitoring Germany's anti-Islam, anti-migrant AfD opposition party.