The Metropolitan Police is facing its most challenging period since Commissioner Sir Paul Stephenson took over, he has said.
Sir Paul, who was deputy commissioner five years ago and took the top post last year, said a combination of violent disorder on the streets, budget cuts, and the forthcoming Olympics were stretching his force.
And he acknowledged the student tuition fees protests were being driven by "a real passion about a cause".
Sir Paul, interviewed on BBC Radio 4's Today programme, was asked how far the protests, cuts, and other events, were stretching his force.
He replied: "When you put all the various challenges that face me and my colleagues in the Metropolitan Police service together, this is as challenging a period as we have ever faced."
He stressed he did not doubt the operational capability of the force. But he said: "I think the challenges we are facing, that stretch we are facing, is not as we have known for a long, long time.
"We can make significant economy savings, but how far we can stretch those savings is a real issue. I am concerned about our current budgetary situation."
On the student protests, Sir Paul said: "There's clearly a sense of anger out there that's led to a level of violence."
He said some protesters were determined to breach police lines and had come prepared, and added: "I think we have got a sense of a real passion about a cause out there. Within that, there are various levels of passion that lead to extremism."Reuse content