A cordon has been lifted at Worthing seafront and two people taken to hospital following a "hazardous material incident".
Part of the seafront was evacuated because of the incident, which left people vomiting and with sore eyes.
A cordon was set up from Windsor Road to Grand Avenue and people were told to stay inside and shut windows and doors.
Police said the two people had been discharged and told to go home, wash their clothes and have a shower.
Police are investigating the source of the chemical incident.
East Worthing and Shoreham MP Tim Loughton wrote on Twitter the investigation was in response to reports of "bathers affected by some sort of chemical type irritants".
West Sussex Fire and Rescue Service said at 4.10pm they had stood down and left the scene, adding that police would remain in the area to reassure the public.
Sussex Police said: "A small number of people have reported symptoms including sore eyes and vomiting, two of whom were taken to hospital. They have both since been discharged, and advised to go home, wash their clothes and have a shower.
"Anyone else affected is advised to do the same - and use copious amounts of water to wash your eyes."
The force added: "We'd like to thank everyone for their patience and understanding during the incident. We are following up a number of lines of enquiry and the investigation into what happened is ongoing."
A staff member at Coast Cafe said people were calm and that "a couple of people were complaining of itchy eyes".
"They've cleared the seafront and we've got the windows closed and are locked in," she added.
Former windsurfing world champion and Olympian Christine Johnston has run Kite, Surf and Sup Co for six years and was on the beach preparing kitesurfing lessons at 12.15pm when she and her students were told to leave by beach patrol.
"They came along on their buggy and said we should leave with immediate effect because there had been an incident," she said.
"They said a few people had been hospitalised. They were covering their mouths with clothing.
"I did ask [what was happening] but they wouldn't comment.
"They told us they were going to go down the area affected, between Lancing and Worthing. We went up to the cafe and saw quite a lot of movement of police cars and an ambulance.
"My eyes were a bit sore and itchy, and a friend who was on the beach said she felt her eyes were sore too."