The attacks have raised tensions in Turkey just three weeks before snap elections are due on November 1 and as the military wages an offensive against Islamic State (IS) jihadists and Kurdish militants.
With the country on edge, Erdogan issued a statement condemning the "heinous" bombings and cancelled a planned visit to Turkmenistan but he has yet to speak in public since the attack that shocked the nation.
On Sunday, thousands of demonstrators thronged central Ankara's Sihhiye Square, close to the blast site by the city's main train station, to pay tribute to the victims.
Many of those gathered accused the government of failing to provide security at the ill-fated rally and several anti-government demonstrators shouted "Erdogan murderer" and "government resign!"
"I am a mother, I'm worried about my grandchildren, I am marching for our children, for our future. Each time there are people dead, I also die a little," said Zahide, who like many others carried a pink carnation flower to commemorate the victims.
The premier's office said 97 people were killed when the bombs exploded just after 10:00 am (0700 GMT) as leftist and pro-Kurdish activists assembled for the rally.
It added that 507 people were wounded, with 160 still in hospital and 65 in intensive care in 19 hospitals.
An AFP correspondent said the scene of the blast was littered with ball bearings, indicating the explosions were intended to cause maximum damage.
- 'Topple the dictator' -
In an emotional address to the mourners in Ankara, the HDP's leader Selahattin Demirtas said that rather than seeking revenge people should aim to end Erdogan's rule, starting with the upcoming legislative elections.
"We are not going to act out of revenge and hatred. But we are going to ask for (people to be held to) account," he added, saying the elections would be part of a process to "topple the dictator."
Even before the attacks, the president was under immense political pressure after his Justice and Development Party (AKP) lost its overall majority in June 7 polls for the first time since it came to power in 2002.
Coalition talks failed and Erdogan called new elections for November 1. But to the disappointment of the AKP, opinion polls show the outcome may be little different to the previous ballot.
The Ankara death toll surpasses that of the May 2013 twin bombings in Reyhanli on the Syrian border that killed over 50 people, making the attack the deadliest in the history of the Turkish Republic.
The attacks drove a knife through the heart of normally placid Ankara, which became the capital following the founding of the modern Turkish Republic by Mustafa Kemal Ataturk.
"This could well be Turkey's 9/11," said Soner Cagaptay, director of the Turkish Research Programme at The Washington Institute.
- Link to IS? -
There has been no claim of responsibility yet for the twin bombings but Davutoglu said groups including IS jihadists, the outlawed Kurdistan Workers' Party (PKK) and the far-left Revolutionary People's Liberation Party–Front (DHKP-C) were capable of carrying out such an attack.
"Work is continuing to identify the corpses of the two male terrorists who carried out the suicide bombings", the office of Prime Minister Ahmet Davutoglu said in a statement late Sunday.
The attack came just under three months after a suicide bombing blamed on IS, also against peace activists, in the border town of Suruc on the Syrian border killed 33 people.
NTV television said the Suruc and Ankara attacks were similar both in style and the type of bombs used. The same forensic experts sent to Suruc are now working in Ankara.
The Hurriyet and Haberturk dailies reported that the elder brother of Abdurrahman Alagoz, who carried out the Suruc suicide bombing, could be implicated in the Ankara blasts.
The Suruc bombing caused one of the most serious flare-ups in Turkey in recent times as the PKK accused the government of collaborating with IS and resumed attacks on the security forces after an over two-year truce.
The military hit back, launching a "war on terror" against the Kurdish militants.
The PKK on Saturday unexpectedly announced it would suspend all attacks -- except in self defence -- ahead of the polls.
But the Turkish army kept up its campaign with more air raids on southeast Turkey and northern Iraq, killing 49 suspected militants over the last two days, the official Anatolia news agency reported.
Two Turkish soldiers were also killed in clashes with the PKK in the eastern Erzurum region, reports said.