They were taken to a central London police station to be kept in custody and interviewed by officers from the Metropolitan Police's counter-terrorism unit.
Searches were carried out at five houses in the Leeds area, in the north of England, and a flat and business property in east London in what the UK police called "a pre-planned, intelligence-led operation".
Fifty-two people were killed when four men – three of them Britons of Pakistani origin and one a naturalised Jamaican – set off devices on three London Underground trains and a double-decker bus. The bombers also died.
The attack, during rush hour, also injured more than 700 people in what was the worst attack on London since World War II.
The police statement said: "This remains a painstaking investigation with a substantial amount of information being analysed and investigated.
"As we have said previously, we are determined to follow the evidence wherever it takes us to identify any other person who may have been involved, in any way, in the terrorist attacks.
"We need to know who else, apart from the bombers, knew what they were planning. Did anyone encourage them? Did anyone help them with money, or accommodation?"
Police said no further details of the men arrested would be released.