Oh they interfere with observers a lot. It's a battle, really.
Sometimes, in some far-flung region, an election will end up being fair. Then they still find a way to ultimately "fix" the outcome — see what happened in Khabarovsk with everyone's favorite mayor Sergey Furgal. They found a court case on him from very long ago, arrested him, flew him to Moscow, then Putin dismissed him from mayorship "for loss of trust". Yeah sure, people elected him and had no problems with him, but it's lack of Putin's trust that caused him to lose his office against his and people's will.
Sometimes, in some far-flung region, an election will end up being fair. Then they still find a way to ultimately "fix" the outcome — see what happened in Khabarovsk with everyone's favorite mayor Sergey Furgal. They found a court case on him from very long ago, arrested him, flew him to Moscow, then Putin dismissed him from mayorship "for loss of trust". Yeah sure, people elected him and had no problems with him, but it's lack of Putin's trust that caused him to lose his office against his and people's will.