A video sent to The Yorkshire Post shows a large group of people congregating in Pearson Park in Hull on Saturday, May 30.
The group can be seen listening to music and socialising in large groups.
Visitors also danced on tables and played on the playground equipment.
An eyewitness who was walking in the park said local residents were in "despair" at the scene.
He said: "This was the scene yesterday in Pearson Park where sound systems and parties were set up.
"Us local residents were in despair once again.
"Nearly every day we see large gangs congregating in the parks with total disregard to social distancing rules.
"It has become a daily occurrence and no one has put a stop to it."
The resident added: "My mother has diabetes and is in the vulnerable category.
"We are doing all we can to help the NHS.
"The park is in the middle of a three million pounds redevelopment but instead we see people partying, dancing on the tables and completely ignoring the rules.
"It is a sad and scary situation."
Although parks reopened in Hull on May 11, people must stick to social distancing rules.
Playgrounds and outdoor gyms remain closed.
An update issued by Hull Council on Wednesday, May 13 said: "Our parks and green spaces currently remain open and there are no plans to close at the moment.
"The latest Government update to guidance on 11 May 2020 means that people can now exercise as much as they want outdoors and travel to places for outdoor exercise as long as they maintain the correct social distancing rules.
"Government guidance on playgrounds and outdoor gyms has not changed, and all of the playground and outdoor gym areas still remain closed as have previously been."
Groups of up to six people will be able to meet in public spaces and private gardens from Monday, June 1.
However, social distancing rules must be maintained and people from different households must stay two metres away from one another.