On her first visit to Britain since husband Barack became US president, Michelle Obama dazzled London with her signature classic style yesterday, leading one newspaper to dub her "Mighty Michelle".

The first lady accompanied her husband to Downing Street first thing for breakfast with British Prime Minister Gordon Brown and his wife Sarah before the men discussed today's G20 summit and the women visited a cancer care center.

Later, she was to meet Harry Potter author J.K. Rowling when she sits next to her at a pre-summit dinner.

Michelle Obama's understated chic has injected a welcome dash of class into the run-up to today's G20 as world leaders descend on London amid rowdy protests in the City of London financial district.

"A razzle dazzling performance from the mighty Michelle," was the London Evening Standard's verdict on her arrival.

After her Downing Street visit, Michelle Obama - dressed in a sparkly cream cardigan and green pencil skirt from one of her favorite shops, J. Crew - and Sarah Brown went to the Maggie's Centre in west London in bright spring sunshine.

There they met patients and their families plus staff, even joining women with cancer for a make-up session.

One woman at the center, Namina Turay, 32, told her she was diagnosed with breast cancer in 2007, but had been in remission for a year.

"Congratulations on being done!" the first lady replied.

Later yesterday, the Obamas were to meet Queen Elizabeth II and Prince Philip at Buckingham Palace before a royal reception for all G20 leaders.

The politicians were then to have a working dinner at Downing Street as Sarah Brown hosts what the British press is calling a "girls' night in", also at the prime minister's official residence.