Leaders from the 27 European Union member states that will remain in the bloc after the United Kingdom leaves will gather next week in Brussels for an extraordinary meeting.
They will chart the course for the organization against a backdrop of mounting concerns about the future of European integration.
European Council President Donald Tusk held a news conference on Friday, seeking to reassure the world that the 27 nations' leaders are united and will consider the way forward for the EU.
But he also expressed deep sympathy for the UK after it became clear it had voted to leave the bloc.
"There's no hiding the fact that we wanted a different outcome from the referendum. I am fully aware of how serious, or even dramatic, this moment is politically," Tusk said.
"There is no way of predicting all the political consequences of this event, especially for the UK. It is a historic moment but, for sure, not a moment for hysterical reactions."
European analysts expressed concern over how the referendum outcome might affect the EU's future.
Fredrik Erixon, director of the European Centre for International Political Economy in Brussels, said the EU will be "consumed" by negotiating the exit of one of its core members during the next two years.
Erixon said the Brexit vote will also strengthen forces in other countries that want to break away from the EU.
"I don't think there is another country that may be on the brink of having a similar referendum, but the Brexit vote will, over the next two years, likely erode the confidence and authority of the EU," he said.
He added that there is a risk that it will push the EU to become less open to trade with the rest of the world, and it is vital now that other countries take a lead in shaping an EU that will not go "in the direction of protectionists".
Paal Frisvold, managing director of the Brussels branch of the Norwegian communication company Geelmuyden Kiese, said: "The Brexit will have an enormous impact on European integration, because it has brought a lot of insecurity and uncertainties.
"Few know how Briton will leave the EU and how this will affect trade, finance, the economy and political instability."