Ukraine's president has said a truce agreed with pro-Russian rebels in the east must be honoured, as he ordered the army to stop fighting.
Petro Poroshenko also warned the rebels against continuing attacks on the besieged strategic town of Debaltseve.
Fighting appeared to have died down immediately after the ceasefire took effect at 22:00 GMT.
But later both the government and the separatists accused each other of mortar shelling in some places.
Earlier on Saturday, rebel leaders had also ordered their forces to stop fighting in line with the ceasefire.
Fighting had raged near Debaltseve just hours before the truce was due to kick in.
Officials say more than 5,400 people have been killed since the conflict erupted in eastern Ukraine in April, but the UN believes the actual death toll to be much higher.
The fighting followed the annexation by Russia of Ukraine's southern Crimea peninsula.
Ukraine and the West accuse Russia of sending troops and weapons to help the separatists in Ukraine's eastern Donetsk and Luhansk regions - a claim the Kremlin vehemently denies.
In a national broadcast from the military headquarters in Kiev, President Poroshenko ordered the commander of what Ukraine describes as an "anti-terror operation" to observe the ceasefire.
Mr Poroshenko expressed hopes that "perhaps the last chance [for peace] won't be lost", adding that agreements "must be honoured, and we are expecting that the ceasefire deal will be adhered to".
The president said that there was still "alarm" over the situation around Debaltseve - a strategic transport hub, where government forces have been for days besieged by the separatists.
The rebels say that they are now surrounding Debaltseve, where several thousand Ukrainian troops are believed to be stationed. Kiev denies the claim.
Donetsk rebel leader Alexander Zakharchenko said earlier on Saturday that he considered the Debaltseve area was not covered by the agreed ceasefire.
The truce deal was brokered on Thursday in Minsk, Belarus by Mr Poroshenko, Russian President Vladimir Putin, French President Francois Hollande and German Chancellor Angela Merkel.
But key issues remain to be settled, including the situation in Debaltseve.
European leaders have warned Russia that it could face additional sanctions if the agreement is not respected.
German Chancellor Angela Merkel said the deal offered "a glimmer of hope", but warned: "It is very important that words are followed by actions."