Two Turkish security sources claimed ISIS were behind the two explosions which killed up to 128 and wounded 247 more at a peace rally in Ankara yesterday.
'All signs indicate that the attack may have been carried out by ISIL (ISIS). We are completely focused on ISIL,' one of the sources said.
Earlier today, Turkish police fired tear gas to disperse mourners who were laying flowers at the site of Turkey's deadliest ever terror attack.
The Queen has offered her 'heartfelt sympathy' to the Turkish people, saying she is 'shocked and saddened' by the attacks.
In a letter to Turkey's President Recep Tayyip Erdogan, 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.'
Protesters clashed with riot police in Istanbul last night as they took to the streets to denounce the attacks. And today, police clashed with demonstrators and pro-Kurdish officials at the scene of the disaster near Ankara's main train station.
They held back the mourners, including the pro-Kurdish party's leaders Selahattin Demirtas and Figen Yuksekdag, insisting that investigators were still working at the site.
Around 70 mourners were eventually allowed to enter the cordoned off area outside Ankara's main train station - where the explosions occurred - to briefly pay their respects for the victims today.
Thousands of mourners then began to march towards a central square in Ankara, chanting anti-government slogans.
Several demonstrators who blamed President Recep Tayyip Erdogan over the bombings chanted 'Erdogan murderer' and 'government resign'.
The rally was organised by labour unions, leftist groups, NGOs and the pro-Kurdish Peoples' Democratic Party (HDP) - the same groups that had called the peace rally targeted in Saturday's attack.
Turkey's Prime Minister Ahmet Davutoglu said ISIS terrorists or far-left Kurdish extremists could have carried out the attack but no group has yet claimed responsibility for it.
His country is vulnerable to attacks by ISIS which controls vast amounts of land in neighbouring Syria.
But the terror group, which is normally vocal on social media about such high-profile attacks, has made no such claim about the Ankara bombing as of yet.
Davutoglu declared three days of mourning yesterday and said there were 'strong signs' the attack was carried out by two suicide bombers.
Investigators have determined that one of the bombers was a male aged about 25 or 30, according to the Yeni Safak newspaper which is said to be close to the government.
Harrowing footage from yesterday morning showed their relatives holding hands and dancing down the streets of Ankara, but joy turned to terror when the first the blast erupted just metres behind them.
The explosion tore through the crowd of people, maiming dozens of innocent bystanders and leaving body parts and debris littering the road.
Pictures which emerged shortly afterwards showed torn fragments of flags and banners people had been waving just moments before littering the ground.
Witnesses described how the blasts, which are believed to have been a terror attack, shook the ground around the city's main train station.