The Queen said she is "shocked and saddened" by the suspected suicide bombings which claimed at least 95 lives in the Turkish capital Ankara.
In a letter to Turkey's president Recep Tayyip Erdogan, the Queen offered her "heartfelt sympathy" to the Turkish people following Saturday's attacks.
She said: "I offer my sincere condolences and heartfelt sympathy to the Turkish people at this time.
"I, along with people across the world, have been shocked and saddened by Saturday's attack in Ankara and my thoughts are with all those affected by these terrible events."
Turkey has declared three days of mourning following the attack, the deadliest in the country's recent history.
There was no immediate claim of responsibility, but Turkish prime minister Ahmet Davutoglu said Kurdish rebels and Islamic State militants were the most likely culprits.
The two explosions occurred seconds apart outside the capital's main train station as hundreds of opposition supporters and Kurdish activists gathered for a peace rally.
The pro-Kurdish HDP party, which organised Saturday's rally, said the death toll could be as high as 128 people.
Television footage showed a line of protesters near Ankara's train station chanting and performing a traditional dance with their hands locked, when a large explosion went off behind them.
At least 248 people were injured in the blasts, and 48 of them are said to be in a serious condition.
Prime Minister David Cameron said he had spoken to Mr Davutoglu to express his condolences.
"My thoughts are with the victims and their families," he added.
Anti-terrorism police in the UK are working with the Turkish authorities.
Assistant Commissioner Mark Rowley, the National Police Chiefs' Council lead for counter terrorism, said: "We are deeply saddened to hear of the two explosions at a peace rally in Ankara that have killed a large number of people.
"Our sympathies are with all those affected, both in Turkey and in our Turkish communities here in the UK.
"The UK's counter terrorism network works in partnership with the Turkish authorities, and our officers are providing ongoing support for our counterparts in Turkey."
At the Vatican, Pope Francis led followers in silent prayer for the victims of what he described as the "terrible slaughter" in Ankara.