David Cameron has entered No 10 as the first Conservative Prime Minister in 13 years.
The Tory leader said he intended to form a "proper and full" coalition government with the Liberal Democrats.
Arriving in Downing Street with his wife Samantha, he announced: "Her Majesty the Queen has asked me to form a new Government and I have accepted."
A large crowd gathered outside Downing St, some booing while others cheered, as Mr Cameron arrived from his meeting at Buckingham Palace.
Mr Cameron paid tribute to Gordon Brown for his "long record of dedicated public service" and said that after more than a decade of Labour rule, Britain was "more open at home and more compassionate abroad".
Looking ahead to the coalition he will form with the Lib Dems, he said: "We have some deep and pressing problems - a huge deficit, deep social problems, a political system in need of reform. For those reasons, I aim to form a proper and full coalition between the Conservatives and the Liberal Democrats."