Moscow authorities have rejected Amnesty International's request for permission to demonstrate in front of the Belarusian Embassy in the Russian capital.
The human rights organization said its activists planned a protest Friday to urge Belarusian authorities to stop executions.
Two Belarusian men, Uladzislau Kavalyou and Dzmitry Kanavalau, were executed with a shot to the head in March 2012.
They were convicted of a Minsk metro-station bombing that killed 15 people and wounded about 300 others, as well as two earlier bombings.
Human rights organizations and independent investigators cast serious doubts about the evidence used to convict the men.
Amnesty International says Moscow authorities offered two other places to hold the demonstration.
The organization rejected those, saying they are in remote locations and have nothing to do with the subject of the protest.