Eyewitnesses said two women were hit when the panel fell on the 07:45 BST Southern service from Tattenham Corner to Victoria.
Katie Burkes, 28, said she suffered bruising and whiplash when the panel fell on to her shoulder. Southern said no injuries were reported.
Southern has apologised and said a full investigation has been launched.
Ms Burkes, from Purley, south London, said: "The woman across from me looked really panicked and then the whole ceiling came down and fell on my shoulder and her shoulder.
"Everyone in the carriage got up and was holding this panel up in the air, and then turned it on its side.
"The driver of the train came on and asked if everyone was ok, but the staff at East Croydon didn't know how to deal with it. There wasn't a manager there and no first aider.
"Southern needs to ensure it's safe. If someone was having a heart attack what would happen? They wouldn't have much chance with no first aider there. [Southern need to] be respectful of how much people are paying for trains."
A Southern spokesman said reports were received of a ceiling panel dislodging in one of the carriages on the 07.45 Southern service from Tattenham Corner to Victoria.
"Southern staff at East Croydon station boarded the train to attend to passengers and it was reported at the time that there were no injuries," he said.
London Ambulance Service confirmed it did not attend the scene.
A ceiling panel has fallen on top of passengers on board a London-bound train, Southern Rail has confirmed.