Families have been invited to the venue in Barker's Pool to help create a giant mosaic to fill the 450 sq m room, using the tiny blocks to make images of astronauts and planets come to life.
The activity is part of Sheffield Bricktropolis - a festival dedicated to Lego that is happening in the city centre until August 30 - and has been organised by the charity Fairy Bricks, which donates Lego sets to children in hospices and hospitals.
Kevin Gascoigne, the organisation's 'chief fairy', said play forms a vital part of sick youngsters' recovery.
“Happy children just feel better more quickly," he said.
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The mosaic was expected to be finished by Wednesday, and is comprised of thousands upon thousands of individual pieces.
“This will have over 101,000 bricks inside of it when it’s finished," Kevin said.
Fairy Bricks are a versatile bunch - as well as donating Lego sets, they can also be hired to build murals and mosaics to make just about any image.
They have carried out work for corporate parties, product launches and other large-scale events; each creation gets displayed but never kept as the Lego pieces are boxed up again afterwards and, in Kevin's words, 'taken back to fairy HQ'. In the past the team have been behind great works such as a Lady Godiva mural in Coventry, and a massive Yorkshire Tea box rendered in the colourful bricks.
Kevin said that, on average, a Lego mosaic or mural can take about three to four days to build, but they never truly know if it will turn out as envisaged until it's fully complete.
Some people, he said, don't even believe their works are really made out of Lego - because they look so similar to a picture until viewers get up close.
Fairy Bricks, which is based in Huddersfield, achieved its biggest Lego mural so far in Denmark last year. The masterpiece, which depicted a comic strip, was made up of 2,585 sections.
The charity started with a single hospital donation in 2012, and now aims to deliver Lego with a retail value of approximately £5,000 per month in the UK and beyond. In 2019 the team has been striving to increase the monthly donations to £7,500 of sets.
At the City Hall, groups of children and their parents sat at 15 tables, following instructions as to how to place the Lego bricks correctly.
Kevin said that, at one table, three generations of the same family were building part of the mosaic.
“They’ve got grandparents, then their children, then their children... this is bringing people together," he said.
To begin, everyone is allocated a bag of bricks to piece together, with a base plate and instructions.
"We were here last year as well but this is a new design and a new event," Kevin said.
The mosaic-building continues next Wednesday and Friday, August 28 and 30, at the City Hall from 10.30am to 3pm. Admission is free and no booking is necessary.
Bricktropolis is funded by the Sheffield city centre Business Improvement District. See www.sheffieldbricktropolis.com for details.