Anti-terrorism police are to begin questioning three men arrested over the 7 July suicide bombings in London.
Two of the 7 July bombers lived in Beeston, Leeds
Two suspects, aged 23 and 30, were detained at Manchester Airport as they prepared to fly to Pakistan while a third, 26, was detained in Leeds.
The arrests, which are the first major ones since the attacks, followed a lengthy police operation.
Fifty-two people died in 2005 after four bombers detonated devices on three London Underground trains and a bus.
Police have been searching five houses in the Beeston area of Leeds, and two premises in east London. The three men will be interviewed at Paddington Green police station in the capital.
Under new anti-terror laws, police can hold them for a maximum of 28 days.
The men were held on suspicion of the commission, preparation, or instigation of acts of terrorism.
Mohammad Sidique Khan, 30, Shehzad Tanweer, 22, and Germaine Lindsay, 19, detonated bombs on three Tube trains and Hasib Hussain, 18, attacked a bus.
Home Secretary John Reid said: "I think the best thing here is not to get ahead of ourselves, not to get into speculation or heighten all of this.
The searches are expected to take some time
"It is a normal part of a very serious and continually ongoing operation and the police will keep everybody informed as is appropriate."
The arrests at Manchester Airport were made shortly before 1300 GMT on Thursday, while the other in Leeds was made just after 1600 GMT.
The addresses of the Leeds searches are in Cardinal Road, Colwyn Road, Firth Mount, Tempest Road, and Rowland Place.
Tanweer and Hussain had both been living in Beeston when the attacks were carried out and Khan grew up in Beeston. Tanweer lived in Colwyn Road with his parents.
The east London searches involve a flat, understood to be in Bromley-by-Bow, and a business, understood to be in Whitechapel.
Scotland Yard said the arrests were part of a pre-planned, intelligence-led operation and also involved the West Yorkshire Police Counter Terrorism Unit.
Ch Supt Mark Milsom, of West Yorkshire Police, said it had not been a high profile operation and unarmed officers were carrying out the searches.
He said the searches may take "some time" but they were not expecting to find firearms or bomb-making equipment.
A Scotland Yard spokesman said: "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?"
BBC correspondent Danny Shaw said that, before Thursday's arrests, the police investigation into the 7 July bombings had been "going on with very little publicity".
The investigation had included a search of a landfill site - "the size of 18 Olympic swimming pools" - at Skelton Grange in Leeds, he said.
Police had "quietly but assiduously" gone through the entire site looking for evidence, our correspondent added.