The arrest was made at the metro station close to the train station
The man was arrested at Snow Hill metro station in the city centre on Friday.
He was reportedly carrying two suitcases which were checked by explosives officers.
The station was evacuated shortly after 1800 BST and reopened two hours later. A spokesman later said the incident was not suspicious.
The bomb disposal team have been and checked and found it to be safe
Insp Phil Milton, British Transport Police
Insp Phil Milton, from British Transport Police, told reporters later that the packages were not found to be suspicious.
"The bomb disposal team have been and checked and found it to be safe," he said.
He revealed that the alarm was raised by an off-duty officer who saw a man and believed his behaviour to be suspect.
The man, said only to be an adult who had travelled from London, was on the tram at the time.
Asked if the arrest was connected to the London bombings, the officer added; "as far as I know its wholly unconnected."
The arrested man has been interviewed by officers but will not charged with any offences.
Part of Snow Hill station is below ground level.
Scotland Yard says the arrest took place at a residence in Stockwell, south London and was "part of the investigation into the incidents of July 21st." The man is being held at Paddington Green police station under the Prevention of Terrorism Act.  Officials will not comment on whether the suspect was one of the four whose pictures were released after being caught by CCTV camera's. 

Just after 6pm BST police arrested a man at the Snow Hill rail station in Birmingham under Section 44 of the Terrorism Act. The station was evacuated and cordoned off because of two suspicious suitcases found at the station.  The suitcases were examined and later found not to contain explosives. The BBC reports that the man who was arrested will not be facing charges.

Snow Hill station re-opened at around 8pm BST.