There is "no easy solution" to dealing with suicide bombers on the Tube, London Underground (LU) managing director Tim O'Toole has said.
Police believe the bombings were suicide attacks
Suicide bombings are a phenomenon to which no one in the world has come up with an answer, he added.
Mr O'Toole also said LU hoped to get Circle Line services back to normal "within days" but it may take longer to restore full Piccadilly Line services.
He also said there were no delays by LU in reacting to last Thursday's attacks.
"We have surveillance techniques, but there is no easy solution to the phenomenon of suicide bombings," he said.
"We consistently review security with other major metro systems in other countries.
"Suicide bombers are a phenomenon that no one in the world knows exactly how to deal with.
"What I can say to our passengers is that they have got to ensure they keep their own belongings with them at all times and not be shy to point out if any bag is unattended."
Mr O'Toole spoke of LU's response to the bombings last Thursday, when initially it appeared that the system had suffered a power surge.
Passenger numbers normal
He said: "We feel that some of what's been said about our response has been insulting. There was no delay in our response.
"We had people leading passengers out of tunnels within two minutes."
The Edgware Road and Aldgate bombings mean there is no Circle Line at present, while the King's Cross incident has resulted in the closure of the Piccadilly Line between Hyde Park Corner and Arnos Grove.
Mr O'Toole said: "The three damaged trains are still where they were.
"We hope that once we get the Edgware Road and Aldgate trains moved, we can have services on the Circle Line running again within days, but it may take longer on the Piccadilly Line."
He went on: "I am sure some people have been put off travelling on the Underground, but passenger numbers on the lines that are running normally this week have been fairly normal.
"I would hope that passengers can take comfort from the professionalism demonstrated by Tube staff."