International leaders will do all they can to ensure the deal struck in Ukraine to end the bloodshed is successful, Prime Minister David Cameron has said.
Opposition leaders, president Viktor Yanukovych and mediators agreed plans for early elections and a new government after bloody clashes between demonstrators and security forces on the streets of capital Kiev left dozens of people dead.
Mr Cameron said the deal offered a "real chance" to bring peace back to the nation and end the "truly shocking" carnage that has spiralled since anti-government demonstrations kicked off three months ago when Mr Yanukovych shelved an association agreement with the EU in favour of closer ties with Russia.
The PM said Russian president Vladimir Putin, German chancellor Angela Merkel and the prime minister of Ukraine's western neighbour Poland, Donald Tusk, all supported the deal.
" I welcome today's agreement which offers a real chance to end the bloodshed and to stop the downward spiral into the nightmare that is facing Ukraine and her people," Mr Cameron said.
"It should foster a lasting political solution to the crisis and President Yanukovych, his administration and the opposition must all get behind this deal and deliver it according to the timetable set out.
"In particular, the 2004 c onstitution must be restored within the next two days and a national unity government should be in place by the beginning of March.
"I hope that it will restore calm to the streets of Kiev and across the country. The scenes we have witnessed over the last few days have been horrific and my thoughts are with those who have lost loved ones and the injured. Such scenes of carnage are truly shocking and have no place in 21st century Europe.
"Over the last 24 hours, I have spoken to President Putin, Chancellor Merkel and prime minister Tusk. We all support this deal and want to see it work. And working with other European partners and the United States, we will do all we can to ensure it succeeds."
The European Union's high representative for foreign affairs, Baroness Ashton of Upholland, welcomed the agreement.
"The purpose immediately must be to see an end to violence," she told reporters in London.
"Implementation is going to be key and very challenging. In everything we now do we need to support the Ukrainian people to implement this agreement. It's a Ukrainian agreement and it should be implemented by them.
"The elements have been discussed in great detail. I think by signing this, president Yanukovych has now staked himself on this."
Lady Ashton said she hoped an association agreement between the EU and Ukraine could now go ahead.
It was Mr Yanukovych's decision to shelve the deal under pressure from Moscow, which originally sparked the protests. Lady Ashton said she believed the EU could now offer the financial assurances he was seeking.
"We have been talking about economic packages short, medium and long term. I have been talking with institutions like the IMF, the European Investment Bank, the European Bank for Reconstruction and Development, all of whom are engaged in the Ukraine," she said.
"The agreement that we have reached is a good one. We would like the country to go forward. President Yanukovych told me many times that's what he'd like to see as well and I hope that that's what will happen."
Ukraine's parliament voted to restore the 2004 constitution and approved measures which could free Yulia Tymoshenko, the former prime minister who has spent more than two years in jail for what supporters say are politically tainted charges.
Foreign Secretary William Hague said: " I welcome today's vote by the Ukrainian Parliament which was overwhelmingly in favour of a return to the 2004 constitution. This is an important first step in implementing the agreement signed this morning between the President and the opposition.
"I urge all sides to adhere to the agreement in good faith to ensure a peaceful political settlement, including a return to the 2004 constitution, a government of national unity and early elections.
"The vote to decriminalise a number of articles under the criminal procedure code is significant, including the possible implications for Mrs Tymoshenko. We need to see progress towards establishing a fairer and more transparent justice system in Ukraine.
"The United Kingdom will work to support these positive developments over the coming days and weeks."