Vandals broke into a horticultural centre and beheaded 18 birds, speared a frog to death and killed fish by pouring fertiliser into a pond.
Police said the level of cruelty shown by the offenders was "shocking and distressing".
The yobs also caused more than £10,000 worth of damage at Wythenshawe Park in Wythenshawe, Manchester, as they chopped down trees and smashed 130 windows.
They broke into the property some time between 4.30pm on Sunday and 10am the next day.
Councillor Mike Amesbury, Manchester City Council's executive member for culture and leisure, said: "This is a sickening act of cruelty and vandalism, causing significant damage to a centre which has been enjoyed by generations of people.
"I know that everything possible is being done to catch those responsible for this appalling crime and bring them to justice."
Pc Terina Wild, of the Northenden Neighbourhood Policing Team, said: "The level of vandalism and cruelty shown by the offenders is shocking and distressing.
"This is an extremely serious offence and I would ask the community to help me identify the culprits.
"Did someone you know come home with filthy clothing or were they acting strangely? If you know who is responsible, but didn't get involved at the time, please come forward. This is an appalling crime which has affected a community facility and the offenders need to be brought to justice.
"Someone will know who is responsible and it is important you get in touch."
Anyone with information should call police on 0161 856 4546, or the independent charity Crimestoppers anonymously on 0800 555 111.Reuse content