Former prime minister Yulia Tymoshenko has stressed Ukraine "must be a member of Nato" in order to protect itself from Russian aggression.
Tymoshenko, who is running in the May 25 presidential election, told the Associated Press that while only a minority of Ukrainians supported Nato membership previously, Russia's actions in the country's east had forced a "fundamental change" in public thinking.
She said Russian President Vladimir Putin "was able to change the mentality of Ukrainians and turn us in a different strategic direction" due to "his war against us".
"Nato is the best choice for Ukraine," Tymoshenko added.
Former Ukrainian prime minister Yulia Tymoshenko has been embraced by her supporters after officially entering the race to be the country's next president.
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.
In a video message recorded by former Ukrainian Prime Minister Yulia Tymoshenko called for an emergency parliamentary session.
Ms Tymoshenko said that parliament should appeal for the UN Security Council meeting to be held in Kiev.
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.
Ukrainian opposition leader Yulia Tymoshenko has accepted an invitation from Chancellor Angela Merkel to be treated for a back problem in Germany.
The former prime minister was released from the hospital where she had been held under prison guard for most of the time since she was convicted of abuse of office in 2011. She was freed hours after the ousting of Viktor Yanukovich as president.
Ukrainian opposition leader Yulia Tymoshenko says she does not want to be considered for the post of prime minister.
Earlier, her supporters in parliament said she was one of the three favourites to be named to head a government of national unity.
"It was a surprise for me when I heard that I was being proposed for the post of prime minister. Nobody agreed this with me or discussed it with me. "I am grateful for the respect this shows, but I ask not to be considered for this post," she said in comments on her website.
This is a revolution that is over - but not yet complete. It is over because Yanukovich cannot and will not come back. Not complete because we do not yet know how the east of the country is going to react.
There does not seem to be any great rising in support of Yanukovich in the Russian speaking east.
As for what comes next, Yulia Tymoshenko has governed this country before, she was widely considered to be both corrupt and pretty incompetent. Her charisma may mean that she gets to rule again here.
But what this country needs more than anything is leadership free of the persistent and endemic corruption that has impoverished this nation.
I'm afraid that even after everything that has happened here, they may not get it.
Ukraine's freed opposition leader Yulia Tymoshenko has spoken to Angela Merkel by telephone, according to Reuters.
The German Chancellor "congratulated Yulia Tymoshenko on her release and expressed the certainty that her return to mainstream politics would become one of the main factors in stabilising the situation in Ukraine", a statement said.