Children will be asked to build models using Lego City and then upload photos of them to the Nickelodeon website, www.nick.co.uk, which contains a clickable map of the British Isles. Children in the same region can view each other's entries by clicking the map.
The winners, one from the UK and one from Ireland, will star in a 40-second TV ad for Lego City and receive free products for their classmates.
Laura Di Bonaventura, senior brand manager at Lego, said: "It is essential that City receives impactful marketing support using the most innovative, creative and engaging means.
"This campaign allows children to literally enter their own City world. It encourages our target audience to interact with the brand and really experience the core elements of creativity and building everyday, instantly-recognisable models."