Volunteers linked to Russian opposition figure Alexey Navalny's team were taken ill on Tuesday.
According to CNN, their 'Coalition Novosibirsk 2020' office was attacked with an unknown substance.
Two masked men in tracksuits and wearing masks ran into the office and threw a bottle containing a yellow liquid with a strong, pungent, chemical smell.
There were around 50 people in the office, including employees, attending a lecture for election observers.
At least three volunteers were sickened, including one who fainted and went into convulsions.
Navalny visited Novosibirsk just a few days before he was taken ill last month. Elections are due to be held in the region later this month.