Britain's Prince WIlliam, the Duke of Cambridge, signs a book of condolences for the Orlando mass shooting victims while his wife Catherine, Duchess of Cambridge, and Matthew Barzun, U.S. ambassador to London, look on at the U.S. Embassy in London, June 14, 2016. Philip Toscano - WPA Pool/Getty Images

As the world mourns the deadliest mass shooting in U.S. history, Prince William and his wife, Kate Middleton, visited the U.S. Embassy in London Tuesday to pay their respects to the 49 people killed early Sunday morning in an Orlando, Florida, gay nightclub. The Duke and Duchess of Cambridge offered their "thoughts and prayers" and signed a book of condolences at the American consulate in the United Kingdom's capital city.

"With our deepest condolences and with our thoughts and prayers to all those affected," William wrote, and Kate simply signed the book with her full name, "Catherine" before both went on to meet with U.S. Ambassador Matthew Barzun and the members of the embassy's staff who have LGBT interests, reported U.K. news outlet

The shooting took place at a gay nightclub.

The royal couple's visit to the U.S. embassy followed thousands of people in London attending a vigil there Monday to honor the shooting's victims, including the 53 people who were injured in the apparent Islamic State-inspired attack. During the vigil — which was attended by newly elected London Mayor Sadiq Khan and centered on a two-minute moment of silence — balloons were released, Reuters reported.

"We stand here today united together in solidarity with our U.S. neighbors, our friends, our colleagues, our family and all of those affected across the LGBT community," said Angela Crawley, the Scottish National Party equalities spokeswoman.

“This hideous and cowardly attack on LGBT people in Orlando is an attack on our freedoms and values," Khan, London's first Muslim mayor, told local news outlet the Standard. The gunman, Omar Mateen, was also a Muslim. "We stand shoulder to shoulder with the people of Orlando,” Khan added.

At the vigil, Khan vowed to protect the city's LGBT community, Pink News reported.

“It’s important to be here tonight because we in London, the LGBT community, has been on the receiving end of murderous homophobic attacks in the past, and we remember those who passed away here in 1999," Khan said in reference to a deadly London bombing that also left dozens injured. “We are making sure we review our security and keep Londoners safe."