In his strongest condemnation yet of rebels who have seized strategic points in towns in the Russian-speaking east, Rinat Akhmetov urged people to unite "for Donbass without weapons".
Akhmetov, a coal and steel magnate who has an estimated 300,000 employees on his payroll and enjoys huge authority in the region, said Ukrainians should stage a "peaceful warning protest" at their companies from noon when sirens would sound across the region.
The initial response to his call, however, was modest.
Kiev's pro-Western authorities, who hailed Akhmetov's move hoping it would trigger a groundswell of support from ordinary workers to change the dynamics on the ground ahead of Sunday's election, may be disappointed.
Separatist rebellions, fuelled by cross-border propaganda from Russia, erupted in the east after Moscow-backed Viktor Yanukovich was toppled by mass street protests in Kiev in February.
In five months of upheaval, which has caused the worst stand-off between Russia and the West since the Cold War, Russia also seized Ukraine's Black Sea peninsula of Crimea.
Ukraine's struggling pro-Western interim authorities see Sunday's election as a vital step in restoring stability and legitimising the new political order despite separatists who are determined to disrupt it, and hostility from Moscow.
Separatists, who have set up checkpoints and barricades in major towns and have held referendums of their own, have indicated they will do all they can to prevent the election taking place.
Election workers in the east and a senior UN official have said separatists are carrying out intimidation and harassment, including abducting election officials to block the poll.