A senior diplomat resigned in protest over the weekend, and pro-European politicians who are competing with Mr. Poroshenko's party in parliamentary elections next month blasted the decision as caving to Russia, which wants Ukraine to give up the deal and remain in its orbit.
The tensions highlight how difficult it will be for Mr. Poroshenko to manage the competing pressures of a Kremlin that isn't backing down and a domestic electorate that wants closer ties to Europe and no concessions to Moscow.
On Friday, Ukraine and the EU agreed to put off implementing a landmark trade deal, which is part of a broader pact aimed at strengthening their ties, after Moscow threatened trade restrictions that would have crippled Ukraine's already limping economy.
A cease-fire in the east, where Russia-backed rebels hold several towns and cities, is still largely holding despite scattered fighting.
A government spokesman said Sunday that Ukrainian troops had repelled an assault on Donetsk airport by 200 pro-Russia rebels.
In Kiev, pro-Western rivals of Mr. Poroshenko's party railed against the president's move to compromise at congresses to announce candidates for snap parliamentary elections scheduled for Oct. 26.
Former Prime Minister Yulia Tymoshenko, who lost a May presidential election to Mr. Poroshenko, said the delay in implementing the EU free-trade part of the pact until 2016 was "a betrayal of national interests."
"There can't be a single day of applying the brakes on our path to Europe," she told a party meeting.
She also called a referendum on potential membership of the North Atlantic Treaty Organization.
She announced that her party's list of candidates would be led by Nadiya Savchenko, a Ukrainian pilot imprisoned in Russia after being captured by separatists in June.
At a congress of his People's Front party, Prime Minister Arseniy Yatsenyuk announced candidates who include hawkish figures, such as Ukrainian Interior Minister Arsen Avakov and former National Security and Defense Council chief Andriy Parubiy.
The party list's top 10 names also include the leaders of two volunteer battalions heavily involved in fighting.
In a speech Saturday, Mr. Yatsenyuk accused Russian President Vladimir Putin of a revanchist agenda.
"He cannot cope with the idea that Ukraine would be part of a big EU family," Mr. Yatsenyuk said.
"He wants to restore the Soviet Union."
Mr. Yatsenyuk accused Russia of wanting to control all of Ukraine and called for NATO membership as the only way for Ukraine to protect itself.
He acknowledged, though, that this was off the table for now.
Russian Foreign Minister Sergei Lavrov slammed Mr. Yatsenyuk's NATO ambitions, which Moscow sees as a threat to its security.
In a television interview broadcast Saturday, he said Ukraine should commit to a nonaligned status.
Mr. Poroshenko, who has adopted a more measured tone against Moscow in recent days, has allied with Kiev Mayor Vitali Klitschko for elections.
Though the two won't take seats in parliament, their party has included members of Mr. Klitschko's UDAR party and prominent pro-Western journalists who helped launch the protests on Kiev's Independence Square.
Mustafa Nayem, one of those journalists, said he hadn't joined the party but was running on its list.
He said he and fellow journalist activists Serhiy Leshchenko and Svitlana Zalishchuk intend to form a grouping with other young, pro-Western reformers running on the lists of an array of parties and make tackling corruption a priority.
"It's time. We have this gap, this window of opportunity, to go inside and change something," Mr. Nayem said at the party conference, wearing a T-shirt decrying corruption with an expletive.
Former President Viktor Yanukovych's initial rejection of the EU deal in November touched off street protests that led to his ouster in February and swept a pro-Western government to power.
Russia then invaded and annexed Crimea, and is backing separatists in Ukraine's east who are demanding autonomy.
Russia is using its huge economic leverage over Ukraine to block its integration with the West.
Ukraine's central bank chief said on Saturday that the country's war-ravaged economy could shrink by 10% this year.
Moscow's threats to ramp up trade restrictions would have wielded an even bigger blow.
The EU has extended Ukraine's reduced import tariffs, and said it hopes postponing the trade deal will help ease tensions between Kiev and Moscow.
But delaying the agreement presents risks for Ukraine.
"It formally acknowledges Russia's veto right over the deal," said Vasyl Filipchuk, a former senior Ukrainian diplomat.
Mr. Filipchuk added the postponement wouldn't prevent further economic pressure from Moscow aimed at bringing Kiev to heel.
Russia has already cut gas supplies amid a payment dispute.
Deputy Foreign Minister Danylo Lubkivskiy said he had handed in his resignation over the delay, which he said sent "the wrong signal to everyone: the aggressor, allies and, most important, Ukrainian citizens."
"A choice isn't postponed, otherwise it's not a choice," he wrote in a Facebook post.
Source: The Wall Street Journal