The president says he's confident Britons, like Americans, "will not be intimidated by thugs and assassins."
His words of support for Londoners came after this month's second wave of attacks on the mass transit system.
The president said no one will "frighten us from our great love of freedom."
As for Mr. Bush's push to privatize Social Security, the president received a little help from his mother.
Former first lady Barbara Bush teamed up with her son the president on Friday in trying to drum up support among older Americans for his Social Security and Medicare plans.
He called his 80-year-old mother "my favorite senior citizen," and she tartly reminded him that at age 59 he was almost one himself with hair starting to turn white.
"You can see where I got my white hair from," the president retorted.
At a senior center, and then before an invitation-only audience at a downtown civic center, the mother and son team promoted Mr. Bush's embattled Social Security restructuring plan and the new Medicare prescription drug program that takes effect Jan. 1.
And like a vaudeville team, they kept stepping on each other's lines and zinging each other.
At the Wesley Woods senior center, both Bushes emphasized the importance of getting seniors to sign up for the new prescription drug plan, part of a Medicare restructuring enacted in December 2003.
As Mr. Bush started to talk up the plan, his mother turned to him and said, "Weren't you going to tell people they ought to ask doctors, lawyers, people they trust whether this is a good deal for them?"
"Yes, I am," he said, repeating what she said, and adding, "This is a good deal."
"It saves them money," Barbara Bush said.
"And save money, yes," said the president. "See? That's why she's here: to remind me what to say."
"And save their lives," he added.
When Mr. Bush said he and his mother would go around the room to shake hands, she pointed to the right and told him, "You go that way."
"She's still telling me what to do," Mr. Bush said, going dutifully to the right.
In his civic center speech, Mr. Bush expressed concern that the benefits of the plan may not be initially appreciated by many of the 42 million eligible Americans who receive Medicare benefits. Literature and forms go out Oct. 1 and enrollment begins Nov. 15.
Low-income seniors -- individuals earning less than $15,000 a year or couples earning less than $19,000 -- would have roughly 95 percent of their prescription drug costs covered under the plan.
Turning to Social Security, Mr. Bush vowed that nothing in his proposal for individual investment accounts would reduce benefits for current retirees or those close to retiring. "Seniors have nothing to worry about. ... What you should be worried about is whether your grandchildren are going to get any checks," Mr. Bush said.
"I'm here because I'm worried about our 17 grandchildren," said Barbara Bush.
At another point, Mrs. Bush told her son she was proud of him.
"I was thinking how great you look in your new brown suit," she said.
Mr. Bush gave her a long look and said, "You turn 80, and you run out of things to say."
That brought groans from the audience, and he quickly amended it to say his outspoken mother indeed still had "a lot to say."
Despite crisscrossing the country for months to promote the plan, his top second-term domestic priority, Mr. Bush has had little success in building public support for it.
Mr. Bush's trip came the same week in which House and Senate leaders threw in the towel on trying to get committee action on the legislation before the August congressional recess.
The president has run into solid Democratic opposition and scattered Republican skepticism about his proposal to let younger workers divert a portion of their Social Security withholding taxes into private stock and bond investment accounts in return for a reduction in future guaranteed benefits.
Mr. Bush has also said Congress should take additional steps to assure the long-range solvency of the national retirement program.