JUDY WOODRUFF: The clock is ticking tonight on Britain's withdrawal from the European Union. London got things started today, nine months after the British people voted to quit the political and economic bloc.

Our chief foreign affairs correspondent, Margaret Warner, begins our coverage.

THERESA MAY, British Prime Minister: In accordance with the wishes of the British people, the United Kingdom is leaving the European Union.

MARGARET WARNER: With that, the British government formally launched the process. First, Britain's ambassador to the European Union delivered official notice to its president in Brussels.

Then, Prime Minister Theresa May addressed Parliament.

THERESA MAY: This is an historic moment from which there can be no turning back. Britain is leaving the European Union. We are going to make our own decisions and our own laws.

MARGARET WARNER: Reaction on the street was as mixed today as it was in last June's referendum, which passed 52 to 48 percent.

MIKE PIPER, Great Britain: I said to my wife, all I want to do before I die is see my country free of the shackles of Europe.

NICOLA GIBSON, Great Britain: No one how this is going to go. It's a gamble. It's a risk. It's a bit like — it's just an uncertain place at moment.

MARGARET WARNER: But the European Council president, Donald Tusk, lamented the occasion.

DONALD TUSK, European Council President: There is no reason to pretend that this is a happy day, neither in Brussels nor in London.

MARGARET WARNER: Then-Prime Minister David Cameron called last year's Brexit referendum, hoping it would fail, and boost his Conservative Party's prospects in the next general election.

But after the surprise result, Cameron stepped down, leaving its execution to May. The U.K. has two years to hammer out the specific of its withdrawal from the 27-nation bloc. But face-to-face talks aren't expected to get under way until late May. And the negotiations promise to be tough.

British Finance Minister Philip Hammond said he expects some give and take, but added:

PHILIP HAMMOND, British Finance Minister: I'm confident, as we have explored over the last nine months with our E.U. partners, that we have a sufficient meeting of minds on this issue, that we will be able to reach a deal that will work for us and work for them.

MARGARET WARNER: As for the U.S., then-candidate Donald Trump praised Brexit. Today, his White House spokesman commented.

SEAN SPICER, White House Press Secretary: Whatever future the U.K.-E.U. relationship looks like, we want the U.K. to remain a strong leader in Europe.

MARGARET WARNER: For the PBS NewsHour, I'm Margaret Warner.

JUDY WOODRUFF: And we will hear from the British ambassador to Washington right after the news summary.