The European Parliament's demands follow the British Prime Minister's invoking of article 50 of the Lisbon Treaty, starting a two-year Brexit countdown.
There was a mood of melancholy outside the British Houses of Parliament as protesters gathered to mourn the end of the United Kingdom's marriage to the European Union.
Inside the House of Commons, British Prime Minister Theresa May announced she had invoked article 50 of the Lisbon Treaty, beginning the formal process of withdrawal from EU membership.
"This is an historic moment from which there can be no turning back. Britain is leaving the European Union. We are going to make our own decisions and our own laws. We are going to take control of the things that matter most to us and we are going to take this opportunity to build a stronger, fairer Britain - a country that our children and grandchildren are proud to call home."
A six-page document spelling out the process was delivered to European leaders.
It gives Britain two years to complete exit negotiations.
European Council President Donald Tusk expressed his disappointment.
"Most Europeans, including almost half the British voters, wished that we would stay together - not drift apart. But, paradoxically, there is also something positive in 'Brexit'. 'Brexit' has made us - the community of 27 - more determined and more united than before."
Going it alone could come at a significant financial cost for the UK.
European Parliament leaders want Britain to settle a bill with the EU that could exceed more than 80 billion (AU) dollars.
While the parliament's chief negotiator, Guy Verhofstadt, says it's still possible for Brexit to be reversed if the union's remaining 27 states agree: "Naturally it will never be the same. It will never be outside the union better than inside the union, and that is not a question of revenge. That is not a question of punishment. That is the logic of the European Union, of the European treaties, of the European project. That together we created an added value for everybody of our countries and for every of our citizens."
German Chancellor Angela Merkel has called on Theresa May to look after Europeans living in her country.
"For many people in Europe, Great Britain's intended departure from the EU is connected to very concrete worries about their own personal future. This goes especially for the many Germans and European citizens living in Great Britain. Therefore, the German government will work intensively to make sure the effect on the everyday lives of those people is as small as possible."
Outside Europe, Australian Trade Minister Steve Ciobo was talking up the opportunities Britain's departure presented.
"We've seen, for example, with the European Union Australia has had a lot of constraints, especially around agricultural exports in the EU with low levels of quotas. I want to make sure that we can use this 'Brexit' opportunity as the opportunity to boost those and basically to make it as straightforward as possible when it comes to goods, services and investment."
In the UK, there were mixed feelings about beginning formal EU divorce proceedings.
This couple in the northern city of Sunderland voted to leave in last year's referendum, telling reporters they regret it now.
"With all the unsatisfaction that it has given us, taken along doing it. All the parties are fighting about it now. They don't know, they don't know themselves what they want to do about it."
"I don't feel comfortable with what we've got ourselves into and I think, we should have another referendum."
But there are no second thoughts from this woman...
"Well in my opinion, bring it on, I think it's a great thing."
Reporter: "Why?"
"Because we should realistically be allowed to make our own decisions as a country which hasn't happened for years and years and years so..."
Reporter: "No regrets for you then?"
"Not at all!"
But on the other side, perhaps the most emotional tribute to the 44-year union came from European Council President Donald Tusk as he received Theresa May's correspondence confirming the split.
"There is no reason to pretend that this is a happy day, neither in Brussels nor in London. There is nothing to win in this process and I am talking about both sides. We already miss you. Thank you and goodbye."
