Firebugs targeted the site, in Brookfield Park close to Fairfax Road, overnight on Saturday. They tried to torch a five-tonne excavator machine after smashing its windows.

The yobs also ripped wiring from a 3.5-tonne dumper truck. In total around £2,000 damage was done to the machinery, which belongs to the Gap Group based at Walton Summit.

But determined Preston Council bosses have vowed the damage will not delay the opening of the all-weather pitch, which should be finished by the end of June.

Brookfield resident Ann Bamber, a community volunteer, said: "The pitch is being constructed for the benefit of the young people and someone has decided to wreck the thing before it's even finished.

"People have worked hard to get that money together. It's a real shame."

A Preston Council spokesman said: "We would urge anyone with information as to who may have done this to contact the police.

"Although this is causing some delays, the contractor is hoping to have new equipment available soon and will be back on site early next week. The play area should be completed by the end of next month."

Sgt Dave Herbert, of Fulwood police, said: "It's a thoughtless act which doesn't take into account that these projects are a benefit for the whole community."

A spokesman for the Gap group said: "The damage is being paid for by the people who contracted us but we still have a five-tonne machine that we will be forced to keep out of use for at least a week."

Brookfield Park was among 10 play areas given a cash injection for improvements after a £529,025 grant towards the city's parks from the Big Lottery fund, coupled with match funding from other sources. Plans were submitted for the all-weather pitch in October 2007.

Anyone with information should call Preston police on 01772 203203 or Crimestoppers on 0800 555111.