Thanks to one blog reader who has mentioned that they were in the park when this happened and were asked to leave. Apparently they were told that the police were probably going to pull something out that would be upsetting for the kids to see but wouldn't say what.
The police apparently have found something that would have upset the kids had they seen it. However contrary to much speculation, it is apparently not a dead body. The park will remain closed (with police guards) overnight and the work will continue in the morning. The park should hopefully be open again later tomorrow.