By Alessandra Prentice
KIEV (Reuters) - Ukraine's presidential election effectively became a two-horse race on Saturday after boxer-turned-politician Vitaly Klitschko pulled out and threw his weight behind confectionary oligarch Petro Poroshenko.
Klitschko's withdrawal sets up a May 25 contest between the man known as the 'Chocolate King' and former prime minister, Yulia Tymoshenko.
Poroshenko, 48, confirmed his candidacy late on Friday. Several opinions polls already had him in the lead even before he said he would run to succeed ousted president Viktor Yanukovich.
Poroshenko was an early and influential supporter of the 'Maidan' popular uprising that toppled Yanukovich in late February, three months after he spurned a deal on closer ties with the European Union and plunged the country of 46 million people into turmoil.
Speaking on Saturday, Poroshenko said the political forces that brought down Yanukovich must stick together to tackle the huge economic and security challenges facing Ukraine.
"I'm convinced it would be a betrayal of Maidan if we were not united," he told a meeting of Klitschko's UDAR (Punch) party, Interfax news agency reported.
"I'm convinced that today the volume and scale of the challenges facing the state ... demand this kind of unity."
Ukrainians will vote under the shadow of Russia's annexation of Ukraine's Black Sea Crimea peninsula.
Like Tymoshenko, who announced her candidacy on Thursday, Poroshenko has promised to strengthen Ukraine's armed forces and protect its borders, which the West fears are still under threat from a possible Russian incursion into Russian-speaking regions of eastern Ukraine.
"We need to build a new, efficient and modern Ukrainian army, which will defend the sovereignty and integrity of our country," Poroshenko said late on Friday as he announced he would run for president.
He officially submitted his candidacy to the election authorities in Kiev on Saturday.
The oligarch, whose net worth was estimated by Forbes at $1.3 billion, is the owner of Roshen, one of the world's top twenty confectionary firms but which has borne the brunt of trade sanctions from Russia since last year.
He is an experienced politician, having held several ministerial posts including a brief stint as economy minister under Yanukovich.
A prominent backer of the 2004-05 Orange Revolution against the election fraud and sleaze of Ukraine's post-Soviet establishment, Poroshenko campaigned for the role of prime minister in its wake, but lost out to Tymoshenko, who co-led the revolution.
Klitschko urged his supporters to back Poroshenko, and announced he would run instead for mayor of the capital. So far, no other candidate is seen mounting a serious challenge to the two frontrunners.
The parliamentary faction Party of Regions, Ukraine's former ruling party, also announced its candidate on Saturday.
Deputies voted for Mykhailo Dobkin, a businessman and former governor of the eastern city of Kharkiv, who fiercely opposed the Maidan uprising.
(Writing by Matt Robinson and Alessandra Prentice; Editing by Sophie Hares)