By David Brunnstrom

LONDON (Reuters) - U.S. Secretary of State John Kerry entered 10 Downing Street with a bang on Tuesday, before expressing amazement at the rapidity with which new British Prime Minister Theresa May had taken up residence.

Kerry, making his first visit to London since Britain voted last month to leave the European Union, hit his head on the door as he entered the official residence.

Apparently unhurt, he later exchanged small talk with the Conservative leader, saying he was "amazed" at the speed of a transition that saw her move in to No.10 last Wednesday - right after David Cameron moved out.

"It happened rather quickly ... It's such a different transition arrangement than in the United States," May told Kerry.

"I am amazed it happens so fast - how do you have time to pack everything?" Kerry replied at a photo opportunity where the two shook hands, smiled and sat down on matching armchairs in front of a period fireplace.

Cameron quit after Britons voted narrowly in a June 23 referendum to leave the European Union, having failed in his bid to persuade them to back remaining in a bloc the United Kingdom joined in 1973.

Emerging from the talks, Kerry said he had an excellent discussion with May: "I am very grateful to her for her restatement of the commitment to the very strong transatlantic partnership," he said.

Without referring directly to the so-called Brexit vote, he said both had affirmed the need "to maximize the economic opportunity, minimize the disruption, deal with this in a way that has the wellbeing of the citizens ... in mind".

The U.S. presidential race, meanwhile, grinds on with Republican Donald Trump due to win the formal backing of his party at this week's convention in Cleveland, Ohio. Rival Hillary Clinton is poised to secure the Democratic ticket next week.

Americans will cast their votes in November and the 45th president, and successor to Barack Obama, will not be sworn in until next January.

An aide to Kerry said he was unhurt in his encounter with the front door of No.10.

The towering 72-year-old, at 6 foot 4 inches the second tallest American politician not to win the presidency, has had a number of mishaps on his travels, including breaking his right femur while cycling in the French Alps in May 2015.

Kerry was unhurt on a visit to India in January 2015 when his limousine was involved in a motorcade shunt caused when a driver braked to avoid a dog that had strayed onto the road. The dog survived.

(Reporting by David Brunnstrom; Writing by Douglas Busvine; editing by Stephen Addison)