Sometimes they are, but (mostly in large cities) people sign up as election monitors to prevent it. Sadly, most of 90 000 polling stations are not monitored by independent observers.
The main way to control elections now is not letting opposition candidates to get into the ballot. For example, on this elections many candidates were rejected because they are "linked" to Navalny's organisation that was admitted as an extremist organization by the court. Some people are rejected because the signatures they have collected are deemed invalid. Obviously you cannot win if you cannot get into the ballot.
There also is electronic voting (voting over Internet) which is difficult to monitor. It is also worth noting that while you can vote over Internet, you cannot collect more than 50% of required signatures for being added to a ballot this way.
The main way to control elections now is not letting opposition candidates to get into the ballot. For example, on this elections many candidates were rejected because they are "linked" to Navalny's organisation that was admitted as an extremist organization by the court. Some people are rejected because the signatures they have collected are deemed invalid. Obviously you cannot win if you cannot get into the ballot.
There also is electronic voting (voting over Internet) which is difficult to monitor. It is also worth noting that while you can vote over Internet, you cannot collect more than 50% of required signatures for being added to a ballot this way.