A man was stabbed, eight people were arrested and serious injuries reported after violent disorder erupted on a city seafront and High Street.
Essex Police said a Section 60 dispersal order would remain in place in Southend-on-Sea until 20:00 BST on Wednesday after the disorder on Tuesday night.
The force said seven weapons were also seized, and a video shared on social media appeared to show at least four people dressed in black brandishing large knives near the resort’s Adventure Island theme park.
The local MP Bayo Alaba said there was “a pattern now” of people posting on social media about planned meet-ups on the beach, which needed to be “looked at differently”.
An East of England Ambulance Service spokesperson said ambulances arrived at the scene at 19:00 BST after reports that a man has been stabbed.
“One patient was transported by air to Royal London Hospital. Essex Police transported another patient to Southend University Hospital,” the spokesperson said.
Two ambulances, two ambulance officer vehicles, four Hazardous Area Response Team (HART) vehicles and the Essex and Herts Air Ambulance were at the scene, they added.
Ch Supt Morgan Cronin, from Essex Police, said: “I’d like to reassure the wider public that these were targeted attacks involving just a small fraction of the visitors to Southend yesterday.
“We will not excuse this dangerous behaviour on our streets, at the expense of the public and our officers.
“These arrests were made quickly to keep the public safe, and we have multiple lines of inquiry to identify more suspects and bring them to justice.”
A spokesperson for the force said: “Motivations behind the incidents reported are being investigated.”
The disorder began at about 19:00 with large crowds of young people seen on the Southend’s seafront, which had been busy on the UK’s hottest day of the year.
Essex Police said an assault with an offensive weapon “was seen by security staff and members of the public in Marine Parade” at about 19:40, with “one suspect arrested on suspicion of possessing a bladed article and causing grievous bodily harm”.
Members of the public were dispersed from the area by about 23:30, prompting additional police patrols in the city overnight.
Mr Alaba, Labour MP for Southend East and Rochford, has posted on X about the disorder and said: “I do want to send out my thoughts to all the people involved; there are some people in hospital with quite serious injuries.”
Mr Alaba added: “This time last year it happened as well; this is a pattern now and because it is a pattern we have to look at it differently and see what we can do to mitigate it.
“I am just speechless looking at the videos and seeing how it unfolded; it is truly alarming.
“We don’t want this on our streets and in our community.”
The Section 60 order allows police officers in uniform to stop and search any pedestrian or anything carried by them, as well as vehicles, for offensive weapons “without reasonable suspicion“.
It covers areas such as Marine Parade, Pier Hill, High Street, Western Esplanade – and both the city’s railway stations.
“We have a zero-tolerance approach to violence, disorder and carrying weapons in Essex,” Ch Supt Cronin said.
The force added: “While our work is ongoing there will be a large and visible police presence across the city centre, and further disorder will not be tolerated.”
At the scene on Wednesday morning, BBC reporter Lewis Adams said: “The seafront at Southend-on-Sea is a much quieter place than it was last night.
“People were seen fleeing from outside Circus Amusements when things turned ugly at the heart of the city.
“All that remains from the frantic scenes that circulated on social media on Tuesday evening are bags of discarded litter.
“This area is so close to Southend Pier that visitors, including families with children, flock to it in their droves throughout the year.”
Martin Richardson, the owner of the resort’s Happidrome Arcade, said he believed the police were anticipating disorder, but that intelligence was “not taken seriously”.
He said a “beach rave” had been organised on TikTok and that people had travelled to Southend for it.
“No knife attack is acceptable full stop, and I hope these perpetrators are caught,” he said.
“It should have been dealt with, these larger groups meet up with other groups.
“This is our front room, we see this time in and time out.”
Waste collector Stuart Arnold, 51, told the BBC: “I think it is ridiculous, it happens every year and it needs to be sorted at the train station.
“You’ve got kids here; I’ve got a three-month-old.
“You bring her here, what could happen while you’re pushing your daughter along in a pram?
“It’s silly; it needs sorting out.”
‘Extremely upsetting’
Daniel Cowan, Labour leader of Southend City Council, told BBC Essex: “These were extremely upsetting scenes.
“We know that a large number of youths came with intent to behave in this deplorable manner.
“It is unclear how many of them came in a ‘spur of the moment’ thing.
“It is deeply concerning.”
He said he was working with Essex Police to understand the causes.
“We do not want this kind of behaviour and we want to stop people like this travelling to Southend,” he added.
“We need to learn how to stop these co-ordinated events.”
Essex Police thanked “the vast law-abiding majority of the public for their co-operation” and asked witnesses to come forward.
Anyone with information or dashcam footage of the disorder has been asked to contact the force on its website or call 101, quoting incident number 1094.