A man and a woman have been detained by specialist officers at Heathrow Airport on the suspicion of "the commission, preparation or instigation of acts of terrorism".
It's understood that the couple were arrested after arriving on a flight into the airport.
Police are searching two separate addresses in East London as part of their inquiry.
One thousand officers a day will be on duty during the Olympics in the biggest safety operation Transport Police in the south have ever staged.
Click below for the full report from our Transport Correspondent, Mike Pearse
A major operation is underway to prevent a terrorist attack on the rail network during the Olympic Games.
For the first time many passengers will see armed British Transport Police at our stations. Mike Pearse spoke to BTP Deputy Chief Constable Steve Thomas about the measures.
A major security exercise is underway at the Port of Dover designed to test the ability of Kent's emergency services to deal with a terror attack.The scenario is that two terrorists have detonated a 'dirty bomb', exposing 50 border control staff to hazardous chemicals.
Hundreds of paramedics, firefighters and police are taking part in a simulation in the port of Dover.
Crews will be cleaning up a simulated chemical spill from a lorry which has accidentally dropped its hazardous cargo.
Officers are keen to stress that this is an exercise so residents should not be alarmed if they hear sirens in the area.