Media mogul Rupert Murdoch has been accused of abusing his power to manipulate democracy by a leading activist in a speech made at the Commonwealth Day service.
Ricken Patel, founding president of the global campaigning movement Avaaz, said empires run by figures like Mr Murdoch should be broken up for the good of all.
The Queen was due to attend the annual Westminster Abbey event but a few hours before guests began arriving Buckingham Palace announced she would miss the celebration as she is still recovering from the symptoms of gastroenteritis.
Mr Patel told the congregation, who included the Duke of Edinburgh and High Commissioners from across the Commonwealth, that nations were threatened by global challenges such as climate change, food security and nuclear proliferation that could bring people together or split them apart.
Mr Patel, who will give the annual Commonwealth lecture this week at the Guildhall, said these potential problems could be opportunities if democracies were more accountable to the aspirations of its people.
The activist, whose organisation aims to close the "gap" between the world people have and the one they want, added: "The age of tyrants is passing but we still have many systems that undermine democratic promise.
"Systems like the corporate capture of the legislature in the United States or the media industrial complex in the UK or Australia, where media barons like Rupert Murdoch abuse their power to manipulate democracy.
"But if we come together as citizens we can overcome these barriers, get big money out of politics and use ownership limits to break up the empires of Murdoch and others like him."
The Queen's absence at the service would have been keenly felt by the guests as she is head of the Commonwealth and takes a keen interest in its affairs. The monarch spent a day in hospital last week being assessed after experiencing symptoms of gastroenteritis, which include vomiting and diarrhoea.
A Buckingham Palace spokeswoman stressed that the monarch would attend a reception on Monday evening to sign the Commonwealth's new charter interpreted by some as backing gay equality in every Commonwealth country as well as enshrining other values and principles.
