"I have ordered (the military) to secure the defense of Mariupolwith howitzers, multiple rocket launchers, tanks, anti-tank weaponsand air cover," Poroshenko told a crowd of steel workers in the port on the Sea of Azov near the Russianborder.
The ceasefire, which took effect on Friday evening, is part of apeace plan intended to end a five-month-old conflict theUnitedNations ' human rights envoy said had killed more than 3,000people. It has also caused the sharpest confrontation betweenRussiaand the West since the Cold War.
The truce was largely holding on Monday, though each sideaccused the other of sporadic shelling, including in Mariupol, acity of about half a million, shortly after the president's arrivalthere.
The Organisation for Security and Cooperation in Europe (OSCE),which is monitoring the ceasefire, urged the two sides to seek apolitical "breakthrough", though they remain far apart on thefuture of eastern Ukraine,home to much of the country's heavy industry. The rebels refuse toaccept rule from Kyiv.
"Mariupol was, is and will be Ukrainian," Poroshenkodeclared.
"The enemy will suffer a crushing defeat," said Poroshenko, whoagreed to the ceasefire and a wider peace plan after the rebels -backed, Kiev says, by Russian firepower - made sweeping battlefieldgains. Russiadenies involvement.
In the earlier days of the uprising, rebels seized control ofpart of Mariupol, occupying some buildings including a policestation. Some offices were badly damaged or burnt down. Since therebels were driven out by Ukrainian forces, sentiment appears tohave swung more in favor of the government.
Residents built fortifications around the town, whose port isvital for Ukraine's steel exports, and set up a militia. Shops have reopened andUkrainian flags are widely visible.
Mariupol was the scene of fierce fighting before the ceasefire,when rebels advanced in an attempt to retake it, and also saw themost serious violation of the ceasefire on Saturday night whengovernment forces there came under artillery attack.
A woman was killed and four people injured in thatshelling.
The cities of Donetsk and Luhansk, strongholds of the rebellion,remain in rebel hands.
Poroshenko, who received a warm welcome in Mariupol, said therebels had so far handed over about 1,200 prisoners-of-war to theUkrainian side under the terms of the ceasefire accord.
OSCE chair Switzerland described the ceasefire as "shaky" andsaid the next few days would be crucial.
Swiss President Didier Burkhalter said the truce alone was notsufficient, adding: "The different actors must really push for a(political) breakthrough."
After his trip to Mariupol, Poroshenko said a number of NATOcountries had agreed on the direct supply of arms to Ukraine duringthe alliance's annual summit in Wales.
"(We) managed to agree with a series of NATO countries on directdeliveries of modern weapons which will help us defend ourselvesand win," said a statement on the president's website.
A senior aide to Poroshenko said on Sunday Kyiv had agreed inWales on the provision of weapons and military advisers from fiveNATO states, but four of the five swiftly denied any such deal hadbeen reached.
U.S. Defense Secretary Chuck Hagel said in Turkey hewas not aware of any "secret deal that was made in Wales aboutsupplying lethal weapons to the Ukrainians".
Earlier, Ukrainian military spokesman Andriy Lysenko told a newsbriefing in Kyiv that Ukrainian forces were observing the ceasefireexcept in self-defense and had remained in their positions sinceFriday evening.
In rebel-held Donetsk, eastern Ukraine's industrial hub, "primeminister" Alexander Zakharchenko of the self-proclaimed DonetskPeople's Republic made similar accusations against the Ukrainianside, adding: "We have shown the whole world we are not terrorists,we are ready for talks and we can listen."
Kyiv and its Western backers accuse Russia of sending troopsacross the border and arming the rebels, charges Moscowdenies.
Lysenko said on Monday Ukraine had seen no sign of Russian troopmovements over the border in the past 24 hours.
A Kremlin statement said Russian President Vladimir Putin spokeby phone on Monday with Poroshenko on "steps that will facilitate apeaceful resolution to the situation", without giving furtherdetails.
Russian Prime Minister Dmitry Medvedev said Moscow would respondto any new Western economic sanctions imposed over its role inUkraine, adding it might target flights over Russia.
The European Union planned to put Russia's top oil producers andpipeline operators Rosneft, Transneft and Gazprom Neft on its listof state-owned firms that will not be allowed to raise capital orborrow on Europeanmarkets, an EU diplomat said.
In general, the EU sanctions on raising money in the EU forRussian companies will apply to firms that have turnover of morethan 1 trillion rubles($26.95 billion), at least half of itgenerated from the sale or transport of oil, the diplomatsaid.
But EU governments delayed signing off on the package becausesome governments want to discuss how to suspend the sanctions if aceasefire holds, diplomats said.
EU sanctions do not encompass the gas sector and in particularstate-owned Gazprom, the world's biggest gas producer and thebiggest gas supplier to Europe.
In Donetsk, authorities declared Monday a public holiday to markthe expulsion of "fascists" from the heavily industrialized, mainlyRussian-speaking Donbass region.
The separatists have used the word "fascist" to denote thecentral government in Kyiv since Poroshenko's predecessor ViktorYanukovich, who came from eastern Ukraine and was backed by Moscow,fled to Russia in February after months of anti-government protestsin the Ukrainian capital.
On Monday afternoon a Reuters reporter heard renewed mortar firenear to the government-held airport north of Donetsk.
Most residents of Donetsk blame Kyiv for the conflict, aftermonths of heavy bombardment of the city by government forces, butsome had harsh words also for the separatists and few expected thecurrent ceasefire to last.
"The ceasefire is not holding, that's clear from just the fewdays I have been back in the city," said Yevgenia, who has takenrefuge with relatives in western Ukraine.
"We came back for warm clothes and are leaving right away. It'sso sad to see the city empty, deserted, armed people with carscrossing at red traffic lights, kidnapping people or taking awaytheir cars. What good can they build here?"
(Additional reporting by Thomas Grove and Timothy Heritage in Moscow, Pavel Polityuk in Kyiv, PhilipStewart in Ankara and Stephanie Nebehay in Geneva; Writing by Gareth Jones; Editing by Andrew Roche)