Why did the protesters make such a protest?

Opposition protesters in the Belarusian capital Minsk have attacked the main government buildings, smashing windows and kicking doors. [9] They are accusing the authorities of widespread vote-rigging in Sunday’s presidential election. Police have been attempting to disperse the tens of thousands of demonstrators, and an opposition presidential candidate, Vladimir Neklyaev, was beaten up shortly before the polls closed. Exit polls suggest that President Alexander Lukashenko will win a fourth term by a wide margin.
    The protests are continuing. They started on one square, October Square, and then they were broken up and they moved down to go on at the other end of the main street, and they are now at Independence Square, and there’s a fear that the riot police may move in and break that one up. They are trying to get rid of Mr. Lukashenko. Their main claim is that the elections were rigged well before the actual voting, [10] but now they’ve moved on and are demanding his impeachment or immediate removal.