Crowds gathered outside Queen Elizabeth II's Buckingham Palace residence in London on Thursday as concern grew over the 96-year-old's health.

Around 100 people braved torrential rain to stand at the palace gates, according to AFP, as senior royals rushed to Balmoral in Scotland, where the queen was being treated.

Doctors have placed the monarch under medical supervision, prompting concern from British political and religious leaders.

Britain's longest-serving monarch has been dogged by health problems since last October that have left her struggling to walk and stand.

"She's been the queen for as long as I've been alive, she's been the queen for as long as my parents have been alive," currency broker Charlie Wolstenholme told AFP.

"So she's really a very, very important part of the fabric. You know, it's going to be terrible (if she dies)."

Many of the well-wishers, some carrying flowers, hailed from outside the UK.

"As a French person, even I am touched by this," said student Chloe Papeil.

"She is a part of English culture, but also global culture."

The queen -- an instantly recognisable figure to billions of people across the world -- is in her Platinum Jubilee year, marking 70 years since she succeeded her father king George VI in 1952.

Israeli doctor Ortal Neeman called her "a very special queen".

"Everybody around the world knows her. I wish for her health, and hopefully she'll always be there."

Royals arrive at queen's Balmoral residence

Royal family members, including Prince William, arrived Thursday afternoon at Queen Elizabeth II's Balmoral residence in Scotland after her health deteriorated, AFP journalists saw.

A car arrived at the castle carrying Prince William, the queen's grandson and second in line for the throne, as well as the monarch's middle son Prince Andrew and her youngest son Prince Edward. 

The queen's heir Prince Charles and his sister, Princess Anne, were already at the residence.