The Metropolitan Police Commissioner Sir Ian Blair said the raids were linked directly to the London bombings and described the operation as "significant".
This afternoon armed police took up positions outside a house in the Burley district of Leeds minutes after what was reported to be a controlled explosion.
Police hunting the terrorists behind last week's bomb attacks had earlier raided five other homes in Leeds.
The operation, the first linked directly to the bombings, was announced shortly after 9am by Scotland Yard.
Sir Ian said: "There have been a series of searches carried out in Yorkshire. Those searches are still going on. There's very little else I can say at the moment, but this activity is directly connected to the outrages on Thursday."
Police have been examining the theory that those responsible for Thursday's attacks could have been "away day" bombers, who arrived in the capital to plant the devices and then fled.
It has also been suggested that the bombers could have met at King's Cross station as all three Tube trains which were hit had passed through there on the morning of the blasts.
The bus which was ripped apart by an explosion 57 minutes after the Tube bombs detonated had also come from that direction.
Trains from Leeds to London arrive at King's Cross station.
Scotland Yard has launched its biggest manhunt yet to catch those responsible for Thursday's bombings.
Hundreds of extra officers have been drafted in to assist the inquiry, which is being led by the Met's Anti-Terrorist branch.
More than 1,700 people have called the anti-terrorist hotline since the attack, some providing specific information which police hope will lead to a breakthrough.
The largest-ever trawl of CCTV footage is also under way, with officers poring over hundreds of hours of tape in the hope that one of the bombers has been caught on camera.
In an interview with BBC London this morning, Sir Ian said it was " likely" there would be another attack but insisted the terrorist threat could be defeated.
"Another attack is likely, there's no doubt about that. But when - who knows?" he said.
"Together with New York we are sister cities joined in resolve but we are both major terrorist targets."
He continued: "Our track record is that there has scarcely been a terrorist outrage in London for which we have not found people responsible.
"This is much more difficult than Irish republican terrorism, but it is still something the communities of Britain can defeat if they join together."Reuse content