An anti-immigration group sparked panic in the centre of Prague after dressing up as Isis fighters and staging a fake terror attack.
The apparent protest, by a group called "We don't want Islam in the Czech Republic", was pre-approved by City Hall officials and known about by the capital's police.
Yet it was allowed to go ahead in a stunt that caused about 80 people, including many from the city's Jewish quarter, to run for cover.
Their shouts of "Allahu Akbar" from an army truck caused one local restaurant owner to pull out his shotgun while others shouted "Islamophobe" when they realised what was happening.
The leader of the group, Martin Konvicka, blamed Muslims for the demonstration going wrong.
"Until a few Muslims started shouting aggressively and pushing toward the performers, everything was going very peacefully," Mr Konvicka wrote on his Facebook page.
Mr Konvicka, who is an entomologist at the University of South Bohemia, confirmed that City Hall officials in Prague had approved the demonstration.
"The police checked our air guns and even counted how many bearded guys were in our group, to make sure we didn't deviate from what we told City Hall we would do," he told the New York Times.
City Hall officials said they would not have allowed the demonstration "had they known" exactly what it entailed.
The group drove into Old Town Square a Humvee truck while waving fake submachine-style BB guns and a black and white Isis flag.
And, in an apparent reference to traditional Middle Eastern animal husbandry, they also had a camel and a goat.
One Jewish restaurant-owner ran to the square with his gun, while children and families knocked over tables and chairs as they fled.
The demonstration was meant to coincide with the anniversary of the invasion of the Czech Republic by the Soviet Army in 1968.
Far-right wing, anti-immigrant activism has increasingly focused on Muslims in the country. Tomio Okamura, an MP who heads opposition movement Dawn of Direct Democracy, told people to walk pigs near mosques as an "instruction for the protection against Islam" in a Facebook post last year.
Tensions have risen since a 25-year-old Czech citizen was charged with trying to join Isis in the first case of its kind in the country this month.
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