Arsenyi Yatseniuk (Party of Change), Vitaly Klitschko (Ukrainian Democratic Alliance for Reform) and Nataliya Korolevska (Social Democratic Party of Ukraine) all said the opposition could win the vote if it showed unity.
But any serious prospect of unity might appear distant as the three of them arrived in Brussels separately and held separate press briefings.
Yatsenyuk, a former Foreign Minister in the government of Yulia Tymoshenko, presented himself as an ally to the iconic opposition leader, who is now serving a seven-year prison sentence on charges of abuse of power.
However, he didn't make the effort to plead for her innocence.
'Nobody's a virgin'
"She is not a virgin. Nobody is, in Ukrainian politics," he said on 24 March, apparently alluding to the fact that most politicians in the country are tainted with graft accusations.
Regarding the political situation in the country, Yatseniuk, who was himself a presidential candidate, blasted President Viktor Yanukovich, the historic leader of the Party of Regions, for opening up the country to Russia, and in particular for the 2010 extension of the lease of the Black Sea military base in Crimea.
"Ukraine will never accept Russian accession, or Crimea’s independence," he said.
He also attacked Yanukovich for having pushed forward changes to the country's Constitution. Indeed, after his election the constitutional court overturned many amendments to the country's constitution, made previously. The Council of Europe's Venice Commission considered that the amendments amounted to a change from a parliamentary system to a parliamentary-presidential one.
Regarding the coming elections, he said he had no illusion that the Party of Regions would resort to massive fraud, as it did during the local elections in October 2010.
Another opposition candidate, Vitali Klitschko, made the case of the need for many international observers for the October elections.
Difficult coalition building
Klitschko is a former boxing world champion, who holds the second best knockout-to-fight ratio of any champion in heavyweight boxing history after Rocky Marciano. Having recently ended his sporting career, he founded in April 2010 the political party UDAR, the Ukrainian Democratic Alliance for Reform, the acronym meaning "punch" in Russian and Ukrainian.
"We're not naïve. The election will not be fair, said Klitschko on 26 March, speaking in English. The former champion, whose political force is credited with 10% of voting intention by opinion polls, is also fluent in German.
Klitschko said he has drawn lessons from the opposition's defeat at the mayoral election in the town of Obukhiv, which took place on 18 March. As UDAR and Tymoshenko's party Batkivshchyna had separate candidates, they were beaten by the Party of Regions.
The former heavyweight boxer said his party had suggested a primary to determine a single opponent, who according to him would have won. The Party of Regions candidate wond 38.6% of the vote, while UDAR and Batkivshchyna together got 40.05%. "Our proposals were ignored," he lamented, adding that the most important was "not to repeat the same mistake in six months".
Nataliya Korolevska, leader of the Social Democratic Party of Ukraine, said yesterday (27 March) that she, Klitscko and Yatsenyuk belonged to a "new generation" of political leaders who had a lot in common and need to unite. The Social Democratic Party of Ukraine, recently renamed "Ukraina Vperyod" (Ukraine forward), gets 3% in recent opinion polls.
Korolevska took to Brussels her campaign for nominating Tymoshenko for the Nobel peace Prize. Her centre-left party was until recently part of the 'Bloc of Yulia Tymoshenko' (BYuT), which has a centre-right orientation. But recent scandals led to the demise of BYut, which is now replaced by Batkivshchyna. The two parties will run separately at the election.
Korolevska did not did not hide her tensions with Oleksandr Turchinov, now in charge of Batkivshchyna since Tymoshenko was imprisoned last year.
"Turchinov is afraid of everything new, he is afraid of reform, he was only seeking for an excuse to expel us," she said, referring to tensions that led to the split, which she called "illegal".
Korolevska said she was hoping that a political union with Yatsenyuk and Klitschko would be possible with the objective of winning the election, and added to the group Anatoliy Hrytsenko, a former defence minister under Tymoshenko, who is now leader of a small opposition party called Civic Position.
She however lamented backdoor deals among opposition leaders, which she said were "similar to the way the ruling party does things".
Asked by EurActiv about the opposition's apparent lack of unity, she admitted that a united opposition force would make it more difficult for the ruling party power to cheat at the election.
The Ukrainian opposition leaders have different European political backing or affiliation. Klitschko enjoys strong support from German Chancellor Angela Merkel's CDU. Korolevska said she was close to the Socialist International. Yatseniuk's Party of Change calls itself "social democrat and social liberal", but has not chosen a European party affiliation yet.