Huge crowds responded to the online open invite, which was also run on other networking websites, and descended on Millennium Square with water pistols, water bombs and buckets for the unofficial annual event.

Council officials say they were horrified at the extent of the damage to the square's showpiece garden where plants were trampled, turf ripped up, and water features emptied and filled with foam. The mechanism for the fountains is also believed to have been damaged.

Adding insult to injury, the parks staff were dismayed to see videos and pictures of people trashing the garden posted online.

Some contributors boasted of how they also targeted pedestrians, cyclists and motorists.

The city council is now intending to supply CCTV footage of the water fight, which wiped out years of work in establishing the garden, to police so officers can establish if it provides evidence of criminal damage.

The gardens were designed to symbolise the city's links with Nelson Mandela and his hometown of Durban and won a bronze medal at the Chelsea Flower Show in 2004.

Coun John Procter, executive member for Leisure, said: "Our parks staff were distraught when they discovered the wreckage. Frankly I'm appalled at the total disregard for people's ongoing enjoyment of this beautiful city centre oasis. To destroy years of careful cultivation for a couple of hours of so-called 'fun' is unforgivable.

"We will be pulling out all the stops to make sure everything is put right. It will stretch parks staff to the limit when they are already working hard on this year's Chelsea garden and the bedding and container planting for the city's summer displays."

Council staff have been assessing the damage and spent much of Tuesday morning jet-washing masses of mud away from the area, a spokesman said.