But the complex is by a busy street, and some lawmakers still wonder whether some of the security was designed with the idea of fending off the type of attacks once mounted by the Irish Republican Army, which in 1979 assassinated a Conservative lawmaker, Airey Neave, using a car bomb in Parliament.

“I am shocked, but I am not surprised,” Chris Bryant, a Labour lawmaker, said of the latest attack. “We have always known that a marauding attack by an individual would be the most difficult to prevent.”

On Thursday, with tightened security, lawmakers crowded into the parliamentary chamber determined to show that they would not be deterred. Ed Miliband, a former leader of the opposition Labour Party, said the mood had been one of “shock and determination and also admiration for the job that the security people are doing.”

“I think we are seeing people’s increased determination to carry on with their normal business,” he said.