Ghani said the attack near the German embassy had not been aimed at a specific target but the whole of the diplomatic area in that part of the city.
Afghan security forces have blamed the Haqqani Network, an al-Qaeda ally, for the tanker bombing, as well as a second deadly attack on a funeral ceremony on Saturday.
Islamist insurgents have been gaining ground in Afghanistan in recent years, but the government is trying to bring the largest such movement, the Taliban, to the peace table, the organisers of the conference said.
In his speech Ghani warned the Taliban that they would not succeed in bringing down the government, and said that the aim of the so-called Kabul Process was to ensure peace with support from neighbouring countries and the international community.
NAN reports that security has steadily worsened since 2014 and the end of the main NATO combat mission, which at its peak featured more than 100,000 American troops and tens of thousands more from alliance partners like Britain.
The current international force in Afghanistan numbers about 13,000, about 8,400 of them are American, mostly tasked with training and advising the Afghan forces.