Neighbourhood and Operation Edge officers in Chorley attended the incident on the Jubilee Recreation Ground in Adlington.
A Lancashire Police spokesman said: "There were reports of around 20 people fighting with one in possession of a hammer.
"As a result PC Seddon has arrested a juvenile male in possession of this hammer so luckily we have taken this off the streets.
"There is a possibility that someone has been attacked with a hammer and we are appealing to anyone who has any information about the incident to get in touch by calling 101 and quoting log reference 0624.
"We need to ensure that the person who has been attacked is safe and well as a weapon of this nature has the potential to cause serious injury."