Alongside her at the multifaith event, held on the second Monday of March each year to mark Commonwealth Day, were Prince Philip, Prince Charles and the Duchess of Cornwall, who dazzled in a purple outfit.
In her speech, which was broadcast simultaneously to all Commonwealth countries, the monarch echoed the theme of this year's day – science, technology and society.
Known to be a keen mobile phone user, she talked about technology's ability to transform lives around the world, but also warned that the internet was an "unaffordable option" for many.
The Queen's Commonwealth Day speech is one of only two in the royal calendar in which she gets to express her own sentiments, rather than those suggested by her advisors. The other is her Christmas Day address.
Among the other guests at the service was Rwandan president Paul Kagame, whom the Queen met in a private audience at Buckingham Palace.
Former Belgian colony Rwanda, which joined the Commonwealth last year, is one of just three member states that wasn't once part of the British Empire.
After the service, Her Majesty attended a reception for dignitaries at Marlborough House alongside Commonwealth Secretary General Kamalesh Sharma.