Britain's new top police officer says the government's controversial control orders for terror suspects are needed to "protect the people of the UK".
Sir Ian Blair is the new head of the Metropolitan Police
The decision to implement them was a dilemma but there was "only one choice", Sir Ian Blair, the new Metropolitan Police chief, said.
The orders, which include house arrest powers, will replace measures allowing suspects to be detained without trial.
The Lib Dems are against house arrests. The Tories will respond on Wednesday.
Mr Blair's comments, on BBC Two's Newsnight programme, come a day after an Egyptian terror suspect held in the UK without trial or charge since December 2001 was freed from jail.
Twelve people held under the original Anti-Terrorism Act 2001 remain in custody in custody in the UK.
Referring to the new control orders, Sir Ian said: "If I have to deploy resources to guard 12 people, I will guard 12 people.
"You would be saying something completely different to me if I said 'no, I'm not going to do that, let them wander the country' and some part of London disappears in a cloud of smoke.
"I'm sorry. It is a dilemma, but there is only one choice."
It was "not for the Metropolitan Police Commissioner to be happy", he added.
"But it is for the Metropolitan Police Commissioner to protect the people of London and the people of the United Kingdom.
"Sometimes there are very hard choices."
Because of a Law Lords ruling that the indefinite detention without trial of foreign terror suspects was unlawful, the only alternatives were to release the suspects or put them under "some kind of control", he added.
The Liberal Democrats will oppose the house arrest plans and unveil their own proposals on Wednesday when the Conservatives will also give their full response.
Earlier, Sir Ian said an attempted terrorist attack on London was inevitable but that the city's diverse communities could help prevent it.
He also pledged to crack down on "middle-class" drug users in London who think it is "socially acceptable" to snort cocaine.
Sir Ian joined the force in 1974 and succeeds Sir John Stevens.