The Kremlin immediately dismissed the peace plan, saying that it looks like an ultimatum and lacks an offer to start talks with the insurgents.
Poroshenko, speaking during his first trip as president to the troubled east, said troops would still fire back if separatists attacked them or civilian residents. He stressed the cease-fire was temporary and would end at 10 a.m. on June 27.
"The Ukrainian army is ceasing fire," he said in a statement carried on his official website. "But this does not mean that we will not resist. In case of aggression toward our troops, we will do everything to defend the territory of our state."
Separatists in the Donetsk and Luhansk regions have declared independence from his government in Kiev, occupied public buildings and fought with heavy weapons against troops from the government in Kiev. Rebel leaders have previously dismissed Poroshenko's plan and it remains to be seen to what extent they would comply — or how much pressure Russia would put on them to cease fire as well.
Russia denies supporting the insurrection and has said that Russians fighting in Ukraine are doing so as private citizens.
The Kremlin said in a statement Friday that an initial analysis of Poroshenko's plan shows that "it's not an invitation for peace and talks, but an ultimatum" to the insurgents in southeastern Ukraine to lay down their weapons. It said the plan "lacks the main element — an offer to start talks."
The Kremlin also said Poroshenko's peace plan was released "deliberately or accidentally" as Ukrainian forces fired on the Russian territory near the border, wounding a Russian customs officer. It said the Russian side was waiting for Ukrainian "explanations and excuses" over the incident.
Russian President Vladimir Putin has voiced concern about the Ukrainian military operation against the rebels but he has resisted both the rebels' pleas to join Russia and repeated calls from Russian nationalists for Putin to send troops into Ukraine.
The White House and European leaders urged support for the plan.
White House spokesman Josh Earnest said "the United States has been very clear in our support for President Poroshenko's effort to bring peace and unity to Ukraine." He added the White House called on Russia to support peace and "to stop supporting the militants who are fomenting so much violence and instability."
Tension between Russia and Ukraine escalated sharply in February when street protests in favor of closer ties with the European Union drove President Viktor Yanukovych from office. Russia denounced the events as an illegal coup involving radical nationalists and annexed the mostly Russian-speaking Crimea region. The rebellion in the eastern regions broke out shortly afterward, with Ukraine accusing Russia of supporting it.
Poroshenko's cease-fire order specified that Ukrainian forces would halt offensive operations at 10 p.m. (2000 GMT, 3 p.m. EDT). The move is intended to give separatists time to lay down their arms and fighters from Russia safe passage to leave the country. The next steps include joint security patrols to prevent crime and looting, new local and parliamentary elections, and steps to protect language rights of the many Russian speakers.
In the longer term, Poroshenko has suggested decentralizing some political authority to the regions and taking steps to secure the language rights of people who speak Russian.
Poroshenko made announcement while speaking with residents in the town of Sviatohirsk in the Donetsk region. It was his first visit as president to the troubled east since being inaugurated June 7. Refugees from the rebel-held town of Slovyansk where some of the worst fighting has taken place were also at the meeting.
Earlier in the day, at the border crossing near Izvaryne in the separatist Luhansk region, an AP reporter saw a line of 100 or more cars waiting for hours to cross from Ukraine into Russia as people fled the unrest. Some of the cars were piled high with possessions. The United Nations said earlier this week that 34,000 people had been displaced by the fighting.
One car had a sign saying "children" on the windshield. A man named Sergei, who would not give his last name for fear of retaliation, said "people are simply leaving everything and trying to escape the war."
The UN says at least 356 people have been killed since May 7 and 34,000 have fled their homes.
Before the cease-fire announcement, Putin's foreign policy aide, Yuri Ushakov, said the Russian president is committed to dialogue on Ukraine and is planning to have a phone conversation with U.S. President Barack Obama in the coming days.
Ushakov also said Putin, on a visit to Austria next week, would be meeting with the chairman of the Organization for Security and Cooperation in Europe to discuss Poroshenko's peace plan.
Dobrnjakovic reported from Izvaryne. Vladimir Isachenkov and Nataliya Vasilyeva contributed from Moscow.
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