Travellers have set up camp near a cross-country course, forcing an event for 2,000 youngsters to be called off.
About 20 caravans have moved on to Western Park, Leicester.
On Saturday Leicestershire Primary Schools Athletics Association was to stage a number of races for youngsters aged seven to 11.
The city council said it was working to resolve the problem but was unable to evict the travellers until welfare checks had been carried out.
May be rearranged
The council's director of environmental services, Adrian Russell, said: "We are very disappointed that travellers are using this site.
"We were working hard to try to avoid cancelling the event and were looking at ways to accommodate it elsewhere, but the organisers decided to call it off."
George Smolinski of the athletics association said the land would have been used as a car park and he found out about the travellers' arrival less than a week before the event.
"We've had to get in touch with about 160 schools to tell them that the event is off because if we haven't got that car park, there's just nowhere for people to go."
The event may be rearranged for November.
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Thousands of children were due to take part in the run at Western Park in Leicester
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