'Already miss you'
"We already miss you .. There is no reason to pretend this is a happy day" - EU President Donald Tusk.
'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.
'Act as one'
"In these negotiations the Union will act as one and preserve its interests" - the EU's other 27 member states.
"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), biggest group in the European Parliament.
"This is a joyous day indeed. Today we formally begin the process of reclaiming the right to decide our own laws, how our taxes are spent, whom we trade with and who gets to come into this country" - eurosceptic British newspaper the Daily Express.
'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."