Among the dead were four UN staff and the International Monetary Fund's top representative in Afghanistan.
Afghan officials are investigating how the three suicide bombers penetrated one of Kabul's most secure districts.
Three police chiefs in the Wazir Akbar Khan district have been suspended over the security breach .. the worst attack on foreign nationals since the fall of the Taliban thirteen years ago.
Correspondent: Michael Edwards, South Asia correspondent
Speakers: Mohammed Shafiq Hamdam, social activist; Janan Musazai, spokesman for the Afghan Foreign Ministry; Ban Ki-Moon, UN Secretary-General
EDWARDS: A group of social activists carrying placards reading "Peace is what we want" and "We will win, terrorism will lose" held a rally outside the remains of the destroyed Lebanese restaurant the day after the attack.
A spokesman, Mohammed Shafiq Hamdam, says it was yet another shocking display of Taliban brutality.
HAMDAM: "The reason we have gathered here today is to remember those who has been killed brutally by terrorists and Taliban here. Those Afghan civilian, those foreign civilians and UN staff who were here to serve for Afghan people and for Afghanistan. Such attack shows the Taliban terrorist has no humanity, has no sympathy and they can attack, they can kill anyone."
EDWARDS: Diners had just sat down to dinner at the popular Lebanese Restaurant when a suicide bomber blew themselves up outside the entrance.
Gunmen then raced through the breach opening fire on those inside, many of whom were foreigners.
Janan Musazai is the spokesman for the Afghan Foreign Ministry.
MUSAZAI: "The Afghan government condemned the terrorist attack in Kabul in the strongest possible terms. This attack only demonstrated the evil face of terrorism to the people of Afghanistan, to the people of the region, and to the people of the international community once again, and it underlines in the view of the Afghan government, the need for the honest and result-oriented cooperation first and foremost of the countries of this region but also of the whole International community in the fight against terrorism."
EDWARDS: At least 21 people died - many of them were foreigners.
Among them were two US civilians, British and Danish nationals as well as the head of the International Monetary Fund in Kabul.
4 United Nations staff were killed.
The Taliban claimed responsibility saying it was in response to a NATO airstrike that killed a number of civilians last week.
The UN Secretary-General Ban Ki-Moon condemned the attack and called for those who carried it out to be punished.
BAN: This is totally unacceptable and this is violation of the international humanitarian law. All the perpetrators must be held accountable. As the United Nations mourns this terrorist attack and victims we remain committed to work for the peace, stability and development of Afghanistan. We fully support the transition of Afghanistan toward better future in peace development and security."
EDWARDS: It is considered to be the worst attack on foreign nationals by the Taliban since the radical islamists were removed from power in 2001.
Western forces are in the process of leaving the country - most will be gone by the end of the year.
Many fear this latest attack could be part of a new Taliban strategy to deter westerners working for non-government and aid groups from coming to Afghanistan.