Donald Trump's state visit to the UK, which could take place in June, is likely to cost millions of pounds to police, Sir Bernard Hogan-Howe said.
Speaking on LBC, the Metropolitan Police Commissioner said there were already "some concerns" about potential protests, but that assessments were continuing.
He said: "At the moment, of course, people are concerned that there might be lots of protests - there have been already.
"So no doubt as the days pass we will make assessments for what is going to happen."
He added: "We cannot definitely say there's going to be huge amounts of problems. I think we have got some concerns already; no doubt we will put a lot of officers out there and keep people safe to make sure that everything goes well.
"And if it is a few thousand officers, obviously it will cost quite a bit of money.
"I would think it is going to be the odd million, but I can't be too precise."
Sir Bernard said he thought the plan was for Mr Trump to visit around June, but he was not sure of exact dates.
"We are just waiting to hear all the details be fleshed out.
"State visits, usually there are two a year, and usually we get about six months' notice, but occasionally it has been far shorter notice than that, and we have just got to get on and do it."