The decision to join the ceasefire was made during a trilateral meeting in eastern Ukraine involving Ukraine, Russia and the OSCE. Rebel leaders, Ukrainian government representatives, Russia's ambassador to Ukraine and an OSCE official participated.
Alexander Borodai, the self-proclaimed prime minister of the separatist Donetsk People's Republic made the ceasefire announcement on Russian state television.
"We agree to suspend all fighting from our side [until Friday]... We hope that over the period of this ceasefire by both sides, we will be able to agree to begin consultations on negotiations on a peaceful resolution to the conflict."
As of 5 p.m. local time, the Ukrainian National Security and Defense Council reported that armed rebels had ceased shooting at government positions in the east of the country.
Another rebel leader said the OSCE observers who have been detained in the region since late May would be released.
Former Ukrainian President Leonid Kuchma, who acted as Poroshenko's representative, said, "I hope that, God willing, we will gradually resolve all the problems."