There has been an outcry in Germany after a far-right protester who targeted a German TV crew was exposed as an off-duty police employee.
The demonstrator in Dresden had demanded the crew stop filming him, and summoned police who then held the journalists for 45 minutes.
The far right was protesting against a visit by Chancellor Angela Merkel.
Germany's justice minister said revelations that the man had a job with the police were "alarming".
Katarina Barley said authorities in the eastern state of Saxony had to investigate urgently what happened, and said press freedom was a vital asset for German society.
What happened in Dresden?
When Mrs Merkel visited the eastern city on 16 August, a crowd of supporters of the far-right AfD party and the anti-Islam Pegida movement turned out, chanting "Merkel must go", "traitors" and "lying press". Lying press (lügenpresse in German) is a far-right slogan that dates back to the Nazis.
At one point a man, wearing a bucket hat in the colours of the German flag, approached a TV crew from public broadcaster ZDF and accused them of breaking the law: "Stop filming me - you're pointing the camera straight at me, you're committing a crime." Filming of protests is allowed under German law.
#Pegidawirkt - Sächsische Polizeibeamte machen sich zur Exekutive von #Pegida / #AfD -Anhängern und behindern TV-Team, das für @ZDF @Frontal21 dreht. Hier ein Auszug, die polizeiliche Maßnahme dauerte ca. 45 min. Zeitungskollegen aus #Dresden berichten von ähnlichen Vorfällen. pic.twitter.com/m1erCDU9WJ— Arndt Ginzel (@GKDJournalisten) August 18, 2018
The protester then brought the police to the scene and the crew was held for around 45 minutes while their press cards were checked and the man's complaint against them was registered.
The police action prompted accusations of interference in the freedom of the press: allegations that were denied by the state of Saxony's centre-right Prime Minister Michael Kretschmer. Police had been the only people acting seriously on the ZDF video, he argued.
It then emerged late on Wednesday in a statement that the man in the hat was an employee with Saxony's criminal police agency.
He was not on duty, was taking part in the protest in a private capacity, and was currently on leave, said the state's interior ministry in a statement.
What will happen to the policeman?
A Dresden police spokesman said on German TV on Wednesday night that its workers came from a cross-section of society, so it was quite possible that the force employed Pegida sympathisers.
However, a senior politician in the liberal FDP party said working for the government was incompatible with taking part in a far-right protest. Wolfgang Kubicki told Focus Online that the man should lose his job.
More broadly, the ZDF reporter who put the video on social media, Arndt Ginzel, accused police of acting as Pegida's "executive" by preventing the work of his TV crew.
He also wanted to know whether the police employee had gone on leave before or after the video of his rant had been published.
ZDF has condemned the actions of police as a clear restriction of reporting freedom.