European Council President Donald Tusk briefs the media after Britain voted to leave the bloc, in Brussels, Belgium, June 24, 2016.? [Photo/Agencies]

Leaders from the 27 EU member states that will remain in the bloc after the UK leaves will gather next week in Brussels for an extraordinary meeting at which they will chart the course of the organization going forward against the backdrop of mounting concerns about the future of European integration.

European Council President Donald Tusk held a news conference on Friday where he sought to reassure the world that the leaders of the 27 countries stand together in solidarity and will start a wider reflection on the future of the union.

But he also showed deep sympathy for the UK after it became clear it will leave the European Union, an organization it joined in 1973.

"There's no hiding the fact that we wanted a different outcome of yesterday's referendum. I am fully aware of how serious, or even dramatic, this moment is politically," said Tusk.

"And there's 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 have also shown great concern about how the referendum outcome might affect the future of the EU.

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 coming two years. He said that will be a massive task and one that will draw a lot of energy from other important initiatives.

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 couple of years, likely erode the confidence and authority of the EU," he said.

"There is a risk that it will push the EU to become less open to the rest of the world, to trade, and it is key now that other countries take leadership in shaping an EU that will not go on the direction of protectionists."

Paal Frisvold, managing director of the Brussels branch of the Norwegian communication company Geelmuyden Kiese, said: "The Brexit will bring enormous impacts to 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."

But European Council President Tusk said he wants to reassure everyone that the EU was prepared for the possibility of the UK deciding to leave.

"As you know, the EU is not only a fair-weather project," said Tusk.

He said he spoke to all EU leaders about the possibility of a Brexit during the preceding two days.

"Today, on behalf of the 27 leaders, I can say that we are determined to keep our unity as 27," said Tusk.

He noted that the procedures for the withdrawal of the UK from the EU are clear and are already set out in treaties.

But Tusk has offered the leaders of the countries that will remain in the EU the chance to have an informal meeting in the margins of the European Council summit next Tuesday and Wednesday, so they can discuss the details of how to proceed.

"And I will also propose to the leaders that we start a wider reflection on the future of our union," said Tusk.

Martin Schulz, president of the European Parliament, said that prior to the summit, parliamentary members will prepare a resolution for Tuesday's meeting after assessing the outcome of the referendum.