Officials said the man did not appear to be targeting Trump or others involved in the trial
Police were called to the busy commuter hub at 11.33 GMT following reports of a suspicious item on board a train, a British Transport Police spokeswoman said.
The station was closed as a precaution while specialist officers assessed the item.
Train operators Southern and Southeastern said on Twitter that trains were not stopping at the station.
"All lines are expected to reopen shortly following the emergency services having dealt with an incident at London Bridge," Southern said in a statement.