They were held by Scotland Yard’s counter terror police at an address in West London and were being questioned while various properties and vehicles were searched.
The men – aged 19 and 20 – were arrested on suspicion of preparing acts of terror.
The older suspect was also the subject of a separate investigation by the South East Counter Terrorism Unit.
Police also suspect him of funding terrorism and failing to disclose information about a terror act.
It is thought the plot was not a random “lone wolf” attack, but something more serious and widespread.
Detectives were searching various addresses and vehicles in West and South East London and the Thames Valley area.
It is thought to be one of only a few arrests this year in the UK of suspects allegedly plotting to attack Britain.
Most recent counter-terror arrests have been of suspects planning to travel to Syria in support of the Islamic State terror group.
Last year police and the intelligence services say they disrupted seven Islamic extremist terror plots against the UK.
Last month Assistant Commissioner Mark Rowley, in charge of the UK’s counter terror police, said his officers were involved in “hundreds of investigations” into terror suspects.
News Source SkyNews