Police in Glasgow moved in to clear Kelvingrove Park after a “street party” to coincide with the royal wedding resulted in “completely unacceptable levels” of drunkenness.

More than 4,000 people, mostly in their teens and 20s, gathered for the unofficial event, which was organised on social networking sites.

Strathclyde Police said 21 people had been arrested.

They said officers came under attack and one was taken to hospital.

The party at the popular park, in the west of the city, was organised on Facebook by two teenagers after Prime Minister David Cameron warned authorities “not to interfere” with street party plans on the royal wedding day.

Glasgow City Council had urged people not to attend what it described as an “unsafe and unofficial” party.

However, organisers of the event set up a sound system with DJs and thousands danced and drank in the sunshine.

The Facebook page said the Kelvingrove Street Party would run from 1100 BST until 2100 BST but police moved in to break it up at about 1700 BST.

Officers on horseback were brought in after police were pelted with bottles.

A police statement said the event was initially good-natured but “as the day progressed a significant number of people became more intoxicated resulting in disorder”.

Ch supt Bernard Higgins said: “It’s really disappointing that on the day of the royal wedding we witnessed the scenes we did.

“At one point my officers came under attack and one was taken to hospital suffering from a head injury.

“He has been discharged and will make a full recovery.”

Ch supt Higgins said his officers would be studying CCTV footage.

He added: “The level of drunkenness was completely unacceptable and frankly irresponsible.”

At least three police vans have had their windows smashed.

About 2,000 people remained in the park for a number of hours despite the police advice to leave.

Eyewitnesses described “chaotic” scenes and sporadic fighting.

This is a reminder of the “lighthearted attitude towards authority” for which Glasgow is famous.