This comes as the official death toll rises to 52.
The four blast sites – three on the underground rail system and one on a bus in Tavistock Square – remained sealed off as investigators carried out their work.
Amid speculation that police had information on a number of suspects, London’s Metropolitan police chief, Sir Ian Blair, urged the public to give officials the time they need to conduct their investigation.
“This is the biggest crime scene in English history, and we’ve got to get this right,” Sir Ian told a news conference outside Kings Cross station.
“We will be absolutely implacable in our determination to find the people responsible for this, and we know we have the support of Londoners and people around the world in attempting to solve this terrible crime,” he said.
Over the weekend, intelligence officials and detectives from about two dozen countries – including Spanish investigators who worked on the Madrid bombing case – met with British officials to discuss leads in the attacks.
Police say each of the four London bombs contained less than 4.5 kilograms of high explosives, about half the weight of the Madrid explosives.
On Saturday London’s Metropolitan Police said the explosives used were most likely “not home-made”, but could not confirm if they were commercial or military in origin.
Meanwhile, speculation has been rife that police are close to identifying a suspect they believe was involved in the attacks.
The Financial Times, citing an unnamed European official involved in the investigation, said progress had been made towards naming who was responsible for the bus blast.
“I think we are going to see photographs of one or more suspects being posted within days,” the official told the paper.
A report in the Daily Mirror said police had placed up to 100 suspects considered the “most likely” to have been involved in the attack under surveillance.
An unnamed intelligence source told the paper that among these were British residents known to have trained in al-Qaeda-type camps overseas.
In the United States, NBC television reported that British investigators had found fingerprints on bomb materials, and that at least four operatives are believed to have been involved. The report has not been confirmed.
Separately, The Times newspaper reported that forensic pathologists were paying close attention to two bodies found inside the wrecked bus to see whether one might have been the bomber.
“There are two bodies which have to be examined in great detail because they appear to have been holding the bomb or sitting on top of it,” an unnamed senior police source told the newspaper.
According to The Times, a single bomb maker using military-grade explosives was most likely responsible for manufacturing all the bombs used in the attacks.
Similar components have been found by police at all four bomb sites, the newspaper said without citing a source.
“The nature of the explosives appears to be military, which is very worrying,” Superintendent Christophe Chaboud, head of France’s anti-terrorism police, who is helping the London inquiry, was quoted as saying.
British police have not confirmed they believe the explosives to be of military origin.