ITV News Europe Editor James Mates wrote on Twitter:
Ukrainian boxer-turned-politician Vitali Klitschko has pulled out of the race for president and chosen to back billionaire confectionery oligarch Petro Poroshenko.
Klitschko's withdrawal, and an announcement by Poroshenko yesterday evening that he would stand in the May 25 election, has drawn the battle lines between the man known as the "Chocolate King" and Ukraine's former prime minister, Yulia Tymoshenko.
"The only chance of winning is to nominate one candidate from the democratic forces," Klitschko told a meeting of his political party.
Vitali Klitschko says war with Russia would be a "disastrous scenario", warning a press conference in Donetsk that Ukraine's army is not in a "good state".
ITV News' Emma Murphy reports:
Leading Ukrainian politician Yulia Tymoshenko has called for Europe to take strong action over Crimea's attempt to join Russia, warning that otherwise Moscow would move to take over the rest of Ukraine and destabilise the continent.
"If we allow Russia on March 16 to hold a referendum at gunpoint, on the annexation of Crimea, we will lose Ukraine, we will lose not only Ukraine but stability in the whole world"; Tymoshenko said.
Ukraine opposition leader Vitali Klitschko, who joined her at a conference in Ireland, said he was hopeful for the future of his homeland.
"We are European geographically, historically and mentally also European, unfortunately we are far away from European standards of living. Despite all of that, we are hopeful and we want to become part of Europe", he said.
Ukrainian politician and likely presidential candidate Vitali Klitschko has called for a "general mobilisation" following the Russian parliament's decision to approve deploying troops in Ukraine's Crimea region.
In a statement Klitschko said: "Parliament must ask the army's commander-in-chief to declare national mobilisation after the start of Russian aggression against Ukraine."
He also asked for the UN Security Council to gather urgently for talks on the crisis.
Vitali Klitschko told ITV News the only way to unite Ukraine was to solve the economic problems and tackle corruption.
Vitali Klitschko says it is a "mistake" for Russia to help former president Victor Yanukovich.
Asked about a press conference the former president is due to hold in Russia later he told ITV News:
"I think it's a mistake for the Russian Federation to help a man like that. A man who shot his people and held all the power.
"Yanukovich has to take responsibility for what has happened in this country. We have a huge political and economic crisis as a result of four years of his rule. He promised to listen to the people and he did not. It's all a result of his total corruption in Ukraine.
"People don't have justice and right now everyone is very upset. People want change. Everyone hopes we can change our country.
"We see right now the result of his power. I told him personally to step back months ago. If he had done that we wouldn't have the huge political crisis we have now in Ukraine."
Vitali Klitschko has condemned the Russian government for their involvement in Crimea and for hosting Ukraine's ousted president Victor Yanukovich.
Mr Klitschko, who will stand in the presidential elections in May, told ITV News this morning:
"The situation in Crimea is very critical. It is difficult to understand why Russia has sent a Government representative to Crimea but we are doing everything we can to keep the situation under control.
"There is a risk the situation could spread and dealing with that is the main task for politicians in Parliament today and also should be for politicians around Europe. We have to let the politicians try and sort this out not the police or military people."
President Viktor Yanukovich has fled Kiev and ministers of the old regime were dismissed.
Here are some of the key players of the crisis in Ukraine.
Ukraine's new parliamentary speaker was granted presidential powers after the ouster of President Viktor Yanukovich.The acting president is an ally of former prime minister Yulia Tymoshenko.
The pro-EU former minister represented the protesters during negotiations with Yanukovich.
The parliamentary leader of the opposition run unsuccessfully for president in 2010.
The former world heavyweight boxing champion is the leader of the pro-European Ukrainian Democratic Alliance for Reforms.
He said he plans to ran for president in 2015.
She led Ukraine during the 2004 "Orange Revolution." After a divisive term as prime minister she narrowly lost to him in an election in 2010.
Tymoshenko was released on Saturday after spending the last two and half years locked up in a hospital under prison guard.
The ousted president is officially wanted by police after being declared "unable to carry out his constitutional obligations" by the Ukrainian parliament. His whereabouts are unknown.