Here are key reactions and commentary following Britain's launch of the process to leave the European Union:
- 'Already miss you' -
"We already miss you... There is no reason to pretend this is a happy day" - EU President Donald Tusk.
- 'Interlinked relationship' -
"The negotiations must first clarify how we will disentangle our interlinked relationship... and only when this question is dealt with, can we, hopefully soon after, begin talking about our future relationship," - German Chancellor Angela Merkel.
- 'Sentimentally painful' -
While Brexit is "sentimentally painful" for the Europeans, it would be "economically painful" for the British. But "it is not our intention to punish (the British) for the principle" - French President Francois Hollande.
- 'Respect' British will -
"We respect the will of the British electorate and her majesty's government in taking steps of departing the European Union. ... Whatever future the UK-EU relationship looks like, we want the UK to remain a strong leader in Europe" - White House spokesman Sean Spicer.
- 'Let's stay friends' -
"The sentence often used in private divorces, 'let's stay friends', rings true in this case. Britain remains our neighbour, like the European Union is for Britain. We need each other" - German Foreign Minister Sigmar Gabriel.
- 'Not leaving Europe' -
"I hope we will be out of the European Union, but we will take part in security cooperation and other forms of cooperation to recognise that while we are leaving the European Union, we are not leaving Europe, we are not giving up on European values" - former British prime minister David Cameron who called the referendum which led to Brexit.
- 'Remains a friend' -
"The United Kingdom remains a friend and ally and economic partner. ... We'll continue to look for ways to create closer trade ties and opportunities for better jobs and economic growth that benefits both of our countries and we will continue to engage as friends and allies" - Canadian Prime Minister Justin Trudeau.
- 'Act as one' -
"In these negotiations the Union will act as one and preserve its interests" - the EU's other 27 member states.
- 'New chapter' -
"Today isn't a good day. marks a new chapter in our Union's history, but we're ready, we'll move on, hoping UK remains (a) close partner" - European Parliament chief Antonio Tajani.
- 'Building new walls' -
"The politicians who fought for were allowed to grow up in a free Europe. Today they are building new walls... History will show that Brexit is a tremendous mistake. It will create a lot of damage for both sides." - Manfred Weber, head of European People's Party (EPP), the biggest group in the European Parliament.
- 'Door schtill open' -
"Dear Brits, ze door is schtill open" - German daily Die Welt, in accented English.
- 'Point of no return' -
"The impossible dream is happening. Today we pass the point of no return," - former UKIP leader and prominent "Leave" campaigner Nigel Farage, who tweeted a picture of the letter being handed over with the phrase "you've been triggered."
- 'Avoiding a messy divorce' -
"The Netherlands are committed to avoiding a messy divorce and building another friendship" with Britain. "We want to prevent others from awarding themselves the biggest parts of the cake... Let's stay calm, be realistic and negotiate" - Dutch Foreign Minister Bert Koenders.
- 'Netherlands will follow' -
"A historic moment! Congratulations Britain, regaining national sovereignty! The Netherlands will follow in a few years time!" - Dutch Freedom Party MP Geert Wilders.
- 'Path still unknown' -
"It is a historic day that will mark Europe's entry onto a path still unknown." I hope that "European citizens will come out with advantages" - Portuguese Finance Minister Mario Centeno.