Protestors began to dismantle the scaffolding and fences for the fair on Victory Square, Bucharest, before being grappled by riot police and pinned to the floor.
Demonstrators chanted “We won’t give Victory Square up,” and “Firea, don’t forget, this square is not yours” as they stopped construction workers from building a Christmas fair at the site of anti-corruption protests.
Activists said on Facebook: “We are asking Bucharest city hall to abandon immediately … the move to fence in Victory square.”
“We urge citizens to protest firmly but non-violently. We must stop this treacherous and rudimentary attempt by mayor Gabriela Firea to discourage protests at a time when they will be crucial for the future of this country.”
Earlier this week Bucharest mayor Gabriela Firea, a senior Social Democrat member, said she would stage a Christmas fair for most of December in the square – a decision which Social Democrat Prime Minister Mihai Tudose said was not “the most inspired.”
Three protestors were taken to a police station, riot police spokesman Georgian Enache told state news agency Agerpres, adding that one of them was accused of hitting another citizen.
There were no further police attempts to stop protesters.
Victory Square was due host to big street protests at the beginning of the year following attempts by the ruling Social Democrats to decriminalise some corruption offences and has been a gathering place for largely peaceful protests since.
Further demonstrations have been announced on social media as parliament gears up to approve a judicial overhaul that has been criticised by centrist President Klaus Iohannis, the European Commission and the US State Department.
Earlier this week, Ms Firea denied accusations she was trying to block protests in a statement and urged people to “put emphasis on the things that unite us, not those that divide.