Three members of a group critical of Czech President Milos Zeman climbed onto the roof of the presidential palace in Prague and replaced his official flag with a huge pair of red underpants.
"The flag of a man who is a shamed of nothing flies at last from the Prague castle," the Ztohoven group of artistic pranksters said on their website and Facebook page.
For the group, the colour red of their boxer-short style protest flag symbolises the unhealthily close link they believe the head of state has with China and Russia.
Zeman's detractors also reproach him for his occasional use of bad language.
Dressed as chimney sweeps, the three men, aged from 33 to 41, clambered onto the roof of the presidential building on Saturday afternoon.
This handout photo made available by Art provocateur group Ztohoven shows their two members dressed as chimney sweeps holding the oversized red underwear.(AFP Photo/ Ztohoven handout)
They have been arrested and could face two years in prison for their actions, police said.
Presidential spokesman Jiri Ovcacek on Twitter decried the "fascistisation" of Prague's cafe culture which "does not shrink at the desecration of state symbols".
Zeman has used the term "Prague cafe" to mock the many intellectuals who supported his conservative rival Karel Schwarzenberg at the 2013 election.
The Ztohoven group has a history of often politically motivated pranking.
In June 2007, the group hacked into Czech public television and aired an image of a nuclear mushroom cloud over a peaceful landscape to illustrate a morning weather bulletin.
The same year, it tampered with 50 traffic lights in the Czech capital, showing comical silhouettes in place of the normal red and green figures "to help passers-by out of their daily routine".