The Queen has called for a united approach to tackle global terrorism during her first speech to the United Nations in New York for 53 years.
She urged all countries to "work together as hard as ever" to fight world problems like climate change in her address to the General Assembly.
The Monarch later laid a wreath at the site of the World Trade Center, destroyed in the 9/11 attacks in 2001.
The trip - her first to New York since 1976 - followed her tour of Canada
Before the Queen spoke, she was welcomed by UN Secretary General Ban Ki-moon, who said that in "a churning and changing world", she was "an anchor for our age".
She then took to the stage, wearing a pale grey jacket and skirt decorated with blue-green flowers, and a matching grey hat.
She said she had witnessed "great change" since her last address to the UN in 1957, "much of it for the better".
But she said: "Many important things have not changed - the aims and values that inspired the United Nations charter endure."
She said the achievements of the UN had been "remarkable", growing from just three overseas missions in 1957 to some 26 now, involving 120,000 staff.
She said, however, that "so much remains to be done", adding: "We are not gathered here to reminisce.
"In tomorrow's world we must all work together as hard as ever if we are truly to be united nations."
The Queen later placed a wreath of flowers at the site of the trade centre's south tower in tribute to the more than 3,000 victims of the 9/11 terrorist attacks.
Accompanied by the Duke of Edinburgh, she chatted to dignitaries and victims' families in sweltering temperatures of 38C.
A large crowd applauded as she walked past to open a British Garden of Remembrance in Hanover Square in honour of the 67 UK victims of 9/11.
BBC royal correspondent Nicholas Witchell said it was a "thoughtful and sombre" end to her tour of Canada and short visit to New York.
He said she had addressed the General Assembly of the UN as the head of state of Britain and 15 other countries, plus the head of the Commonwealth.
The governments of all 16 "realms" were consulted about the content of the speech and the Queen herself had taken a more than usual "hands-on interest", our correspondent added.
Buckingham Palace described the speech as one of her most important in recent years.