Prime Minister David Cameron has praised the "magnificent service" given by the Queen, as she celebrates 60 years on the throne.
He said the monarch's Diamond Jubilee should celebrate her "experience, dignity and quiet authority".
Mr Cameron, the 12th prime minister to serve under the Queen, called her a "source of wisdom and continuity".
The Queen became monarch on 6 February 1952, on the death of her father, George VI.
The prime minister at the time was Sir Winston Churchill.
The Diamond Jubilee will be marked by several events in the coming months, with the main celebrations taking place in June.
Mr Cameron said: "Today is a day to pay tribute to the magnificent service of Her Majesty the Queen.
"With experience, dignity and quiet authority she has guided and united our nation and the Commonwealth over six varied decades."
Some people characterised the monarchy as simply "a glittering ornament, a decoration on our national life", he said.
He added: "That misunderstands our constitution and it underestimates our Queen. Always dedicated, always resolute and always respected, she is a source of wisdom and continuity.
"All my life, and for the lives of most people in this country, she has always been there for us. Today, and this year, in the 60th anniversary of her reign, we have the chance to say thank you."
Labour leader Ed Miliband said: "For our country and our Commonwealth this is a day to pay tribute to the wise, tireless and trusted presence of our Queen.
"She has been a source of stability and security in a changing world during her 60 years on the throne providing guidance and comfort to all she meets.
"The Diamond Jubilee in June will be a chance to celebrate her reign. Today we should thank her for six decades of service."
Deputy Prime Minister Nick Clegg said the Queen had been a "centre of calm and stability" in a Britain that had changed dramatically over the last 60 years.
He added: "And yet all who meet her, no matter how intimidated or star-struck, will tell you that she put them at ease within moments with good humour and her extraordinarily approachable manner...
"I would also like to pay tribute to her immense kindness and warmth."
And Scotland's First Minister, Alex Salmond, said: "I congratulate Her Majesty on the 60th anniversary of her accession to the throne, during which she has served her country with the utmost grace and distinction.
"Her dedication and commitment has inspired countless people across the country and around the world.
"And, of course, Her Majesty's affection for Scotland is reciprocated by Scots of all generations."