A police watchdog investigation has begun after a man was arrested and ended up seriously ill in police custody the next morning.
Mr Reynolds was arrested during a taxi journey after the party
Garry Reynolds, 39, from Southwick, is critically ill in hospital, a police spokesman said.
Mr Reynolds spent Saturday night at a party in Brighton, and was arrested later during a taxi journey. He was found ill in a police cell on Sunday.
The Independent Police Complaints Commission (IPCC) is investigating.
Police are also investigating events between him leaving the party and his arrest.
Officers said it was understood Mr Reynolds spent Saturday evening at a private party at the Rock public house in Rock Street, Kemp Town.
At about midnight, he is thought to have left the party, walking towards Chesham Road.
Officers said he was picked up by a taxi driver in Eastern Road at about 0150 GMT on Sunday.
At about 0235 GMT, the taxi driver stopped in West Street to ask for assistance from police officers on duty there and Mr Reynolds was arrested.
A Sussex Police spokesman said Mr Reynolds was taken to Brighton Custody Centre and placed in a cell.
Later on Sunday, shortly after 1100 GMT, Mr Reynolds was found to be seriously ill.
Sussex officers are looking for witnesses who saw Mr Reynolds between leaving the party and being picked up by the taxi driver. The IPCC wants to hear from people who saw what happened in West Street.
Urging people to come forward, IPCC Commissioner Nicola Williams said the incident happened at a time when "the town centre was busy with revellers".
Mr Reynolds is described as 6ft 1in, thin, with ginger hair, and a missing front tooth. He is said to have a "blown" pupil, which makes one of his eyes appear to be entirely white.
He may have been wearing a charcoal grey full length crombie coat.