Nationalism and populism are on the rise in the Balkan nation, where many remain strongly Russophile despite Moscow's invasion of Ukraine.
The immense 1,110-square-metre flag, meant to symbolise Bulgaria's territory of 111,000 square kilometres, was hoisted up the record-breaking 111-metre (364-foot) pole last week in a forest meadow in the Rhodope mountains.
With it Bulgaria takes the flagpole crown from Finland -- which has a 100-metre one -- though it does not nearly touch the heights of some outside Europe which are almost twice as high.
"This won't make Bulgarians richer but it will raise people's spirits," said Simeon Karakolev, 45, the organiser of an historic annual folk festival at Rozhen, whose foundation is behind the project.
Karakolev raised 500,000 euros ($560,000) in donations for the pole in a campaign embraced by Bulgaria's pro-Russian President Rumen Radev.
Local media said that a number of public companies were approached on Radev's behalf to donate money for the mast on the mountain meadow where the festival is held.
- Marred by controversy -
The campaign was widely mocked on the social media with one meme depicting the president swinging on the pole going viral, while many said a country wracked with high emigration and a crumbling health system had more urgent concerns than collecting funds for a massive flag mast.
Political scientist Ognyan Minchev lamented how patriotism had been hijacked by "leaders who measure national pride by the height of a flag mast... quasi-nationalists dominated by Russian propaganda."
A recent Open Society Foundation study found Bulgaria was among the EU countries most susceptible to Russian propaganda and disinformation.
The concrete poured for the flagpole's foundations on the pristine mountain meadows, and alleged irregularities with its permits, also sparked calls from environmentalists to ban it.
A petition against it collected thousands of signatures.
Karakolev said this was "undeserved hate" and thanked authorities for not "backing down to pressure as checks showed everything is perfectly legal."
Radev slammed "dishonourable attempts to denigrate and break this initiative" as he and Karakolev inaugurated the structure with shouts of "Long live Bulgaria!"
Several thousand people of all ages, many in national costumes, gathered for the ceremony ahead of the three-day annual festival, taking the chance to touch the gigantic flag before it went up.
"Yes, some people don't like it... (but) there are flags in every country. They are one of the symbols of a nation," said reserve army colonel Dimitar Mitev, 69, adding that he hoped this initiative would boost patriotism.
Others were less positive.
"I felt unwell when I saw this rod sticking out of the ground in the middle of the meadows and the surrounding forests. This is human interference in nature," business consultant Sofia Botusharova, 38, from the nearby town of Chepelare told AFP.
Still, thousands of phone screens lit up the night as the cheering crowd waited for the flag to be hoisted. But when the big moment came there was disappointment as the lack of a breeze left it hanging listlessly from the pole.
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© Agence France-Presse