The group of diplomats includes the ambassadors to Belarus from Britain, Italy, Latvia, France and Germany.
More people are continuing to join the protesters whose number has now reached 800.
Although temperatures have plunged to -5° Celsius (23°F), and it is snowing, opposition supporters plan to spend another night on the square.
Belarusian opposition groups defied freezing temperatures to continue their unauthorized protest into Monday, when some 300 people remained on Oktyabrskaya Square through the night, setting up tents despite temperatures plummeting as low as -10° Celsius (14°F).
Thousands of people gathered on the square Sunday to support opposition candidate Alexander Milinkevich.
Official reports indicate that incumbent President Alexander Lukashenko won re-election securing 82.6% of the vote, and Milinkevich garnered 6%.
The opposition insists the results are flawed and is calling for an election re-run.
U.S. officials and the OSCE, Europe's main monitoring organization, said they did not recognize the elections as valid, although neighboring Russia said the elections were fair.
