The release of Ukraine's former prime minister Yulia Tymoshenko has been welcomed by European Union foreign policy chief Baroness Ashton.
In a statement she said the EU is "following minute by minute the rapidly changing political situation in Ukraine".
Meanwhile, William Hague called for the country's political leaders to respond calmly as dramatic events unfolded in Kiev, including Ms Tymoshenko's return to the capital after two-and-a-half years in prison.
The Foreign Secretary said UK and EU allies will support a new government in Ukraine ''as and when that is formed'', as power appeared to have slipped from president Viktor Yanukovych.
Ukrainian MPs voted to oust Mr Yanukovych - who left the capital for the security of his powerbase in the east of the country - and hold early presidential elections on May 25.
Mr Yanukovych described the events as a coup, insisted he was the ''legitimately elected president'' and said he would not step down.
Protesters, curious locals and journalists from around the world flocked to the vacated presidential compound to see for themselves Mr Yanukovych's opulent villa, complete with a collection of historic cars, a wooden galleon and a private zoo.
Addressing the crown in Kiev's Independence Square, Ms Tymoshenko paid tribute to the protesters who had lost their lives in the unrest, saying ''heroes don't die, they are always with us and they always will be our inspiration''.
Speaking from a wheelchair amid occasionally chaotic scenes in the square, which has become the focal point for the opposition movement, Ms Tymoshenko said the protests should continue.
''Until you finish this job and until we travel all the way, nobody has the right to leave,'' she said.
''Because nobody could do it - not other countries, nobody - could do what you have done. We've eliminated this cancer, this tumour.''
In her statement Lady Ashton said: "The European Union is following minute by minute the rapidly changing political situation in Ukraine. I call on all sides to continue engaging in a meaningful dialogue to fulfil the legitimate democratic aspirations of the Ukrainian people.
"The European Union expects everyone in Ukraine to behave responsibly with a view to protect the unity, sovereignty, independence and territorial integrity of the country. I call on everyone to abide by the rule of the law and the Constitution.
"We need a lasting solution to the political crisis. This must include constitutional reform, the formation of a new inclusive government and the creation of the conditions for democratic elections.
"Following the agreement reached by the sides on 21 February, we remain fully committed to support an inclusive political process, help de-escalate the situation and to assist Ukraine in the process of reform.
"These steps could deliver a stable, prosperous and democratic future for all Ukraine's citizens.
"Furthermore, I welcome today's release of former Ukrainian Prime Minister Yulia Timoshenko after more than two and a half years in detention. This comes as an important step forward in view of addressing concerns regarding selective justice in the country. "
Mr Hague and his German counterpart Frank-Walter Steinmeier held talks aimed at pushing for financial support for Ukraine.
A Foreign Office spokeswoman said: ''One of the things they talked about was the economic situation and putting together a financial package which will help to stabilise the situation in Ukraine, to enable the Ukraine to receive long-term support from the IMF.''
A US State Department spokesman said: ''Going forward, we will work with our allies, with Russia, and with appropriate European and international organisations to support a strong, prosperous, unified, and democratic Ukraine.''
With Mr Yanukovych refusing to accept the will of the country's MPs, fears mounted that the country could split in two - a Europe-leaning west and a Russian-leaning east and south.
''They are trying to scare me. I have no intention to leave the country. I am not going to resign, I'm the legitimately elected president,'' Mr Yanukovych said in a televised statement.
''Everything that is happening today is, to a greater degree, vandalism and banditry and a coup d'etat,'' he said.
''I will do everything to protect my country from breakup, to stop bloodshed.''