U.S. President Barack Obama calls Britain's new Prime Minister David Cameron from the White House in Washington May 11, 2010. (Xinhua/Reuters Photo)
"I reiterated my deep and personal commitment to the special relationship between our two countries -- a bond that has endured for generations and across party lines, and that is essential to the security and prosperity of our two countries, and the world," Obama was quoted as saying.
Obama was followed by German chancellor Angela Merkel, who congratulated Cameron on his victory and also invited him to visit Germany.
"Only those who have held the office of prime minister can understand the full weight of its responsibilities and its great capacity for good," Brown said in his resignation speech, concluding his tenure a little short of three years and ending the longest spell of ruling by his center-left Labor party.
Following four straight general election defeats at the hands of Margaret Thatcher's Conservatives, Labor won the 1997 election and also the next two in 2001 and 2005 under Tony Blair, who was prime minister from 1997 to 2007 before he stepped aside to allow Brown into Downing Street.