Opinion polls put him behind ex-prime minister Yulia Tymoshenko, who rose to international prominence in 2004 during the anti-corruption Orange Revolution demonstrations and announced her own run last week.

"This feeling of deep responsibility towards my country... prompted me to decide to run again for the office of president of Ukraine," Poroshenko told a rally in Kiev ahead of the March 31 poll.

Poroshenko asked voters for a mandate to "guarantee the process of European and Atlantic integration, to guarantee our independence, to renew the territorial integrity of Ukraine".

His years in power have seen Kiev seek closer ties with the West as war rumbles on in eastern Ukraine against Russia-backed separatists.

Addressing roughly 1,000 people at the gathering of supporters, Poroshenko stressed the need for peace with Ukraine's powerful neighbour.

Poroshenko has been accused of not doing enough to tackle corruption, but he said the worst of the country's economic struggles was behind it, adding: "I hear a lot of harsh criticism and I take that on board."

In third place in opinion polls is Volodymyr Zelensky, a comedian who once played the role of president in a Ukrainian TV series but has no political experience.

No candidate has an unassailable lead, with Tymoshenko, Poroshenko and Zelensky polling on roughly 16, 14 and nine percent respectively.

Poroshenko was elected in 2014 after his pro-Russian predecessor Viktor Yanukovych was ousted following a wave of popular protests.

The 53-year-old chocolate tycoon promised to pivot the ex-Soviet country of nearly 45 million people towards Europe and has sought to push through ambitious reforms.

But critics say corruption is still rampant and Poroshenko has done little to rein in fellow oligarchs, even if the economy is showing signs of recovery following a recession.

Kiev's relationship with Moscow remains in dire straits.

After the 2014 uprising, Moscow annexed Crimea and supported Russian-speaking separatists in the industrial east, in a conflict that has claimed the lives of more than 10,000 people.

The war has been a huge burden for Ukraine's economy, with Poroshenko forced to rely on assistance from the West.

In December the International Monetary Fund confirmed it would give Kiev a $4 billion, 14-month loan.

Poroshenko's low popularity ratings got a boost after he oversaw the creation of a Ukrainian Orthodox Church independent of Moscow.

His standing also increased after Russia seized three of Kiev's navy vessels and two dozen sailors as they tried to pass from the Black Sea to the Sea of Azov in November.

Russian President Vladimir Putin accused Poroshenko of provoking the naval crisis in a bid to increase his popularity ahead of the vote.