Melitopol, in southeastern Ukraine, was one of the first cities to fall after Russia's invasion in February. It is one of four regions to hold referendums that Kyiv says are a sham.
Fedorov said he believed the main reason for holding the referendums was to enable Moscow to conscript Ukrainians following Russian President Vladimir Putin's announcement of a partial mobilization last week.
"The voting takes place in front of assault rifles, (Russian) men with weapons," Fedorov said.
"People are grabbed (by Russian soldiers) right on the street and forced to vote, not only for themselves but for their whole families. When they (the soldiers) enter a rental house with the tenant inside, they make him vote for every person registered in that particular building."
Fedorov said that passage through the Vasylivka crossing, the only official way to reach territory controlled by Kyiv, had been closed to men aged 18-35 for four days and was completely shut on Sunday.
Reuters was unable immediately to independently verify the status of the crossing or the situation in Melitopol.
The referendums on becoming part of Russia were organised hastily after Ukraine recaptured large swathes of the northeast. Russia's parliament could move to formalise the annexations within days.
