The men, aged 39 and 55, were picked up by officers from Scotland Yard's counter terrorism command.
Both are being held under the Extradition Act after being arrested at a residential address in west London.
More top news
A one minute silence has been held at the Queen's Garden Party in memory of the 22 people killed and 59 injured in the Manchester attack.
Mancunians have come together in defiance after a suicide bomber killed 22 people at a concert.
Extra armed police will be deployed to Wembley and Twickenham this weekend, police have confirmed.