The Belarus parliamentary election was not free, impartial or democratic, the Organization for Security and Cooperation in Europe (OSCE) said on Monday.
"This election was not competitive from the start," Matteo Mecacci, OSCE special coordinator, said.
"A free election depends on people being free to speak, organize and run for office, and we didn't see that in this campaign."
With 109 of 110 races decided, the Belarusian opposition failed to win any parliamentary seats in voting on Sunday, the Central Electoral Commission said.
The OSCE, which had 330 observers, said many prominent politicians "remained in prison or were not eligible to register because of their criminal record".
"We were here in 2010 when some of those people were first arrested and put in jail, and we are sad that their voices could not be heard this campaign," Mecacci said.
Voter turnout was 74.3% overall. The highest turnout - 80 percent - was recorded in Vitebsk region and the lowest - 60 percent - was in Minsk.
The commission declared the elections valid at 3 p.m., after more than 50 percent of eligible voters had cast their ballots.
Representatives of five opposition groups and parties had denounced the vote as opaque and undemocratic, citing interference in the electoral process by the authorities. The commission countered by saying the opposition had resolved long before the election to try to discredit the vote in order to pander to a Western audience.