For 26 years, Vladimir Markovic ran his family transportation business in the heart of Belgrade's Savamala district.

Then, two months ago, it was demolished without warning overnight.

His office was one of several buildings abruptly knocked down on April 24 by mysterious masked men, making way for a massive riverside development that aims to transform the Balkan city.

Authorities say the $3 billion (2.7-billion-euro) project, led by Abu Dhabi-based developer Eagle Hills, will generate thousands of jobs and much-needed investment, as well as revamping rundown parts of the banks of the River Danube for public use.

The grand plans include the Western Balkans' biggest shopping mall, a 200-metre-tall (656-feet) Dubai-style tower, high-end hotels, office blocks and luxury apartments.

Activists carry their symbol of protest against a Belgrade riverside development project - a giant yellow duck Andrej Isakovic, AFP/File

But to opponents in the protest movement "Ne Davimo Beograd" (Don't Drown Belgrade), the shady April demolitions epitomise all that is wrong with the scheme.

"For the last few years we have been trying to warn the public that what's happening with the project is really done... in a non-democratic way and a corruptive, violent way," said Ljubica Slavkovic, a leading protester and architect.

Witnesses quoted by local media said balaclava-wearing men tied up onlookers and took their mobile phones as bulldozers demolished the buildings, while police refused calls for help.

Markovic said he was never contacted about his office being knocked down, then "all of a sudden we found rubble," he told AFP.

Vladimir Markovic ran his family transportation business in the heart of Belgrade's Savamala district until it was demolished without warning Andrej Isakovic, AFP

Prime Minister Aleksandar Vucic initially branded the masked men "idiots" for their behaviour, but said the buildings had been scheduled for demolition.

Six weeks later, under growing pressure, he said "top city authorities" were responsible and would be held accountable, but he stopped short of giving any names.

- Growing protests -

The affair has emboldened protests against the waterfront project, with thousands taking to the streets in recent weeks demanding the resignations of those responsible for the night-time incident.

A computer-generated image of Belgrade's riverfront project, photographed in June 2014 Andrej Isakovic, AFP/File

Protesters also allege a lack of public consultation and hasty changes to planning laws for the project, which is set to transform a two-million square metre area over a 30-year period.

They fear an arts and nightlife hotspot in Savamala could be at risk, and that citizens will end up footing much of the project's bill.

According to the contract, Eagle Hills will invest only 300 million euros, half of which is a loan to the Serbian government for preparing infrastructure.

"Public goods and public resources are being taken to be given, to be used, as a profit for a group of individuals who are in power or close to those in power," said Slavkovic.

"Everything is being done behind closed doors."

Demonstrators protest against the Belgrade riverside development project on June 11, 2016 Andrej Isakovic, AFP/File

Premier Vucic has repeatedly rejected concerns about the project, which is a top priority of his government. He accuses opponents of being "paid from abroad" to destroy it as part of a campaign against him.

Eagle Hills did not reply to AFP's repeated requests to comment.

One of the first phases of the project, a riverside promenade stretching over 1,000 metres, is already being enjoyed by cyclists, walkers and rollerbladers.

"We really insisted a lot... the first things that should be done in this project should not be the big towers, big shopping malls, et cetera but it should be the public areas," said Milutin Folic, Belgrade's director of city planning.

"We want the city and the citizens to have benefits."

- Opening up riverside -

Folic said the project was the culmination of a decades-old plan to remove railway infrastructure and open up brownfield areas, allowing the city to stretch down to its riverside.

"We are just adding a little bit of investment to finish what was started a long time ago," he said, insisting historical buildings would be preserved and increase in value.

Sceptics nevertheless fear that the flashy architectural proposals are inconsistent with Belgrade's character.

An association of prominent Serbian architects has issued a declaration urging the government to stop the project, warning that "there is no identity there".

But Folic, also an architect, dismissed such concerns as a matter of taste.

"Although I would maybe do it differently, I personally think that this project and this plan fit perfectly," he said.